Add swap journaling and group undo/redo with extensive tests.

- Introduced SwapManager for sidecar journaling of buffer mutations, with a safe recovery mechanism.
- Added group undo/redo functionality, allowing atomic grouping of related edits.
- Implemented `SwapRecorder` and integrated it as a callback interface for mutations.
- Added unit tests for swap journaling (save/load/replay) and undo grouping.
- Refactored undo to support group tracking and ID management.
- Updated CMake to include the new tests and swap journaling logic.

.junie/guidelines.md

@@ -1,6 +1,6 @@
 # Project Guidelines
 
-kte is Kyle's Text Editor — a simple, fast text editor written in C++17.
+kte is Kyle's Text Editor — a simple, fast text editor written in C++20.
 It
 replaces the earlier C implementation, ke (see the ke manual in
 `docs/ke.md`). The
@@ -43,7 +43,7 @@ The file `docs/ke.md` contains the canonical reference for keybindings.
 
 ## Contributing/Development Notes
 
-- C++ standard: C++17.
+- C++ standard: C++20.
 - Keep dependencies minimal.
 - Prefer small, focused changes that preserve ke’s UX unless explicitly
   changing
@@ -55,3 +55,4 @@ The file `docs/ke.md` contains the canonical reference for keybindings.
   for now).
 - Inspiration: kilo, WordStar/VDE, emacs, `mg(1)`.
 
+

Buffer.cc

@@ -8,6 +8,7 @@
 #include <string_view>
 
 #include "Buffer.h"
+#include "SwapRecorder.h"
 #include "UndoSystem.h"
 #include "UndoTree.h"
 // For reconstructing highlighter state on copies
@@ -390,6 +391,8 @@ Buffer::insert_text(int row, int col, std::string_view text)
 	if (!text.empty()) {
 		content_.Insert(off, text.data(), text.size());
 		rows_cache_dirty_ = true;
+		if (swap_rec_)
+			swap_rec_->OnInsert(row, col, text);
 	}
 }
 
@@ -443,6 +446,7 @@ Buffer::delete_text(int row, int col, std::size_t len)
 		row = 0;
 	if (col < 0)
 		col = 0;
+
 	const std::size_t start = content_.LineColToByteOffset(static_cast<std::size_t>(row),
 	                                                       static_cast<std::size_t>(col));
 	std::size_t r         = static_cast<std::size_t>(row);
@@ -462,16 +466,19 @@ Buffer::delete_text(int row, int col, std::size_t len)
 			break;
 		// Consume newline between lines as one char, if there is a next line
 		if (r + 1 < lc) {
-			if (remaining > 0) {
 			remaining -= 1; // the newline
 			r         += 1;
 			c         = 0;
-			}
 		} else {
 			// At last line and still remaining: delete to EOF
-			std::size_t total = content_.Size();
-			content_.Delete(start, total - start);
+			const std::size_t total  = content_.Size();
+			const std::size_t actual = (total > start) ? (total - start) : 0;
+			if (actual == 0)
+				return;
+			content_.Delete(start, actual);
 			rows_cache_dirty_ = true;
+			if (swap_rec_)
+				swap_rec_->OnDelete(row, col, actual);
 			return;
 		}
 	}
@@ -479,8 +486,11 @@ Buffer::delete_text(int row, int col, std::size_t len)
 	// Compute end offset at (r,c)
 	std::size_t end = content_.LineColToByteOffset(r, c);
 	if (end > start) {
-		content_.Delete(start, end - start);
+		const std::size_t actual = end - start;
+		content_.Delete(start, actual);
 		rows_cache_dirty_ = true;
+		if (swap_rec_)
+			swap_rec_->OnDelete(row, col, actual);
 	}
 }
 
@@ -488,15 +498,18 @@ Buffer::delete_text(int row, int col, std::size_t len)
 void
 Buffer::split_line(int row, const int col)
 {
+	int c = col;
 	if (row < 0)
 		row = 0;
-	if (col < 0)
-		row = 0;
+	if (c < 0)
+		c = 0;
 	const std::size_t off = content_.LineColToByteOffset(static_cast<std::size_t>(row),
-	                                                     static_cast<std::size_t>(col));
+	                                                     static_cast<std::size_t>(c));
 	const char nl = '\n';
 	content_.Insert(off, &nl, 1);
 	rows_cache_dirty_ = true;
+	if (swap_rec_)
+		swap_rec_->OnInsert(row, c, std::string_view("\n", 1));
 }
 
 
@@ -508,11 +521,14 @@ Buffer::join_lines(int row)
 	std::size_t r = static_cast<std::size_t>(row);
 	if (r + 1 >= content_.LineCount())
 		return;
+	const int col = static_cast<int>(content_.GetLine(r).size());
 	// Delete the newline between line r and r+1
 	std::size_t end_of_line = content_.LineColToByteOffset(r, std::numeric_limits<std::size_t>::max());
 	// end_of_line now equals line end (clamped before newline). The newline should be exactly at this position.
 	content_.Delete(end_of_line, 1);
 	rows_cache_dirty_ = true;
+	if (swap_rec_)
+		swap_rec_->OnDelete(row, col, 1);
 }
 
 
@@ -527,6 +543,12 @@ Buffer::insert_row(int row, const std::string_view text)
 	const char nl = '\n';
 	content_.Insert(off + text.size(), &nl, 1);
 	rows_cache_dirty_ = true;
+	if (swap_rec_) {
+		// Avoid allocation: emit the row text insertion (if any) and the newline insertion.
+		if (!text.empty())
+			swap_rec_->OnInsert(row, 0, text);
+		swap_rec_->OnInsert(row, static_cast<int>(text.size()), std::string_view("\n", 1));
+	}
 }
 
 
@@ -541,10 +563,15 @@ Buffer::delete_row(int row)
 	auto range = content_.GetLineRange(r); // [start,end)
 	// If not last line, ensure we include the separating newline by using end as-is (which points to next line start)
 	// If last line, end may equal total_size_. We still delete [start,end) which removes the last line content.
-	std::size_t start = range.first;
-	std::size_t end   = range.second;
-	content_.Delete(start, end - start);
+	const std::size_t start  = range.first;
+	const std::size_t end    = range.second;
+	const std::size_t actual = (end > start) ? (end - start) : 0;
+	if (actual == 0)
+		return;
+	content_.Delete(start, actual);
 	rows_cache_dirty_ = true;
+	if (swap_rec_)
+		swap_rec_->OnDelete(row, 0, actual);
 }
 
 

Buffer.h

@@ -370,6 +370,71 @@ public:
 	}
 
 
+	// Visual-line selection support (multicursor/visual mode)
+	void VisualLineClear()
+	{
+		visual_line_active_ = false;
+	}
+
+
+	void VisualLineStart()
+	{
+		visual_line_active_   = true;
+		visual_line_anchor_y_ = cury_;
+		visual_line_active_y_ = cury_;
+	}
+
+
+	void VisualLineToggle()
+	{
+		if (visual_line_active_)
+			VisualLineClear();
+		else
+			VisualLineStart();
+	}
+
+
+	[[nodiscard]] bool VisualLineActive() const
+	{
+		return visual_line_active_;
+	}
+
+
+	void VisualLineSetActiveY(std::size_t y)
+	{
+		visual_line_active_y_ = y;
+	}
+
+
+	[[nodiscard]] std::size_t VisualLineStartY() const
+	{
+		return visual_line_anchor_y_ < visual_line_active_y_ ? visual_line_anchor_y_ : visual_line_active_y_;
+	}
+
+
+	[[nodiscard]] std::size_t VisualLineEndY() const
+	{
+		return visual_line_anchor_y_ < visual_line_active_y_ ? visual_line_active_y_ : visual_line_anchor_y_;
+	}
+
+
+	// In visual-line (multi-cursor) mode, the UI should highlight only the per-line
+	// cursor "spot" (Curx clamped to each line length), not the entire line.
+	[[nodiscard]] bool VisualLineSpotSelected(std::size_t y, std::size_t sx) const
+	{
+		if (!visual_line_active_)
+			return false;
+		if (y < VisualLineStartY() || y > VisualLineEndY())
+			return false;
+		std::string_view ln = GetLineView(y);
+		// `GetLineView()` returns the raw range, which may include a trailing '\n'.
+		if (!ln.empty() && ln.back() == '\n')
+			ln.remove_suffix(1);
+		const std::size_t spot = std::min(Curx(), ln.size());
+		return sx == spot;
+	}
+
+
 	[[nodiscard]] std::string AsString() const;
 
 	// Syntax highlighting integration (per-buffer)
@@ -471,6 +536,9 @@ private:
 	bool read_only_                   = false;
 	bool mark_set_                    = false;
 	std::size_t mark_curx_            = 0, mark_cury_ = 0;
+	bool visual_line_active_          = false;
+	std::size_t visual_line_anchor_y_ = 0;
+	std::size_t visual_line_active_y_ = 0;
 
 	// Per-buffer undo state
 	std::unique_ptr<struct UndoTree> undo_tree_;

CMakeLists.txt

@@ -4,13 +4,13 @@ project(kte)
 include(GNUInstallDirs)
 
 set(CMAKE_CXX_STANDARD 20)
-set(KTE_VERSION "1.5.9")
+set(KTE_VERSION "1.6.1")
 
 # Default to terminal-only build to avoid SDL/OpenGL dependency by default.
 # Enable with -DBUILD_GUI=ON when SDL2/OpenGL/Freetype are available.
 set(BUILD_GUI ON CACHE BOOL "Enable building the graphical version.")
 set(KTE_USE_QT OFF CACHE BOOL "Build the QT frontend instead of ImGui.")
-set(BUILD_TESTS OFF CACHE BOOL "Enable building test programs.")
+set(BUILD_TESTS ON CACHE BOOL "Enable building test programs.")
 set(KTE_FONT_SIZE "18.0" CACHE STRING "Default font size for GUI")
 option(KTE_UNDO_DEBUG "Enable undo instrumentation logs" OFF)
 option(KTE_ENABLE_TREESITTER "Enable optional Tree-sitter highlighter adapter" OFF)
@@ -298,13 +298,32 @@ if (BUILD_TESTS)
     add_executable(kte_tests
             tests/TestRunner.cc
             tests/Test.h
+            tests/TestHarness.h
+            tests/test_daily_driver_harness.cc
+            tests/test_daily_workflows.cc
             tests/test_buffer_io.cc
+            tests/test_buffer_rows.cc
+            tests/test_command_semantics.cc
+            tests/test_kkeymap.cc
+            tests/test_swap_recorder.cc
+            tests/test_swap_writer.cc
+            tests/test_swap_replay.cc
             tests/test_piece_table.cc
             tests/test_search.cc
+            tests/test_search_replace_flow.cc
+            tests/test_reflow_paragraph.cc
+            tests/test_undo.cc
+            tests/test_visual_line_mode.cc
 
             # minimal engine sources required by Buffer
             PieceTable.cc
             Buffer.cc
+            Editor.cc
+            Command.cc
+            HelpText.cc
+            Swap.cc
+            KKeymap.cc
+            SwapRecorder.h
             OptimizedSearch.cc
             UndoNode.cc
             UndoTree.cc
@@ -312,6 +331,9 @@ if (BUILD_TESTS)
             ${SYNTAX_SOURCES}
     )
 
+    # Allow test-only introspection hooks (guarded in headers) without affecting production builds.
+    target_compile_definitions(kte_tests PRIVATE KTE_TESTS=1)
+
     # Allow tests to include project headers like "Buffer.h"
     target_include_directories(kte_tests PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
 

Command.cc

@@ -761,6 +761,15 @@ cmd_quit_now(CommandContext &ctx)
 static bool
 cmd_refresh(CommandContext &ctx)
 {
+	// C-g is mapped to Refresh and acts as a general cancel key.
+	// Cancel visual-line (multicursor) mode if active.
+	if (Buffer *buf = ctx.editor.CurrentBuffer()) {
+		if (buf->VisualLineActive()) {
+			buf->VisualLineClear();
+			ctx.editor.SetStatus("Visual line: OFF");
+			return true;
+		}
+	}
 	// If a generic prompt is active, cancel it
 	if (ctx.editor.PromptActive()) {
 		// If also in search mode, restore state
@@ -1970,28 +1979,75 @@ cmd_insert_text(CommandContext &ctx)
 	ensure_at_least_one_line(*buf);
 	std::size_t y  = buf->Cury();
 	std::size_t x  = buf->Curx();
-	std::size_t ins_y = y;
-	std::size_t ins_x = x; // remember insertion start for undo positioning
 	int repeat     = ctx.count > 0 ? ctx.count : 1;
+	std::size_t cx = x;
+	std::size_t cy = y;
 
+	// Visual-line mode: broadcast inserts to each selected line at the same column.
+	if (buf->VisualLineActive()) {
+		const std::size_t sy = buf->VisualLineStartY();
+		const std::size_t ey = buf->VisualLineEndY();
+		const auto &rows     = buf->Rows();
+		UndoSystem *u        = buf->Undo();
+		std::uint64_t gid    = 0;
+		if (u)
+			gid = u->BeginGroup();
+		(void) gid;
+
+		std::string ins;
+		if (repeat == 1) {
+			ins = ctx.arg;
+		} else {
+			ins.reserve(ctx.arg.size() * static_cast<std::size_t>(repeat));
+			for (int i = 0; i < repeat; ++i)
+				ins += ctx.arg;
+		}
+		for (std::size_t yy = sy; yy <= ey; ++yy) {
+			if (yy >= rows.size())
+				break;
+			std::size_t xx = x;
+			if (xx > rows[yy].size())
+				xx = rows[yy].size();
+			if (!ins.empty()) {
+				buf->SetCursor(xx, yy);
+				if (u)
+					u->Begin(UndoType::Insert);
+				buf->insert_text(static_cast<int>(yy), static_cast<int>(xx), std::string_view(ins));
+				xx += ins.size();
+				if (u) {
+					u->Append(std::string_view(ins));
+					u->commit();
+				}
+			}
+			if (yy == y) {
+				cx = xx;
+				cy = yy;
+			}
+		}
+		if (u)
+			u->EndGroup();
+		buf->SetDirty(true);
+		buf->SetCursor(cx, cy);
+		ensure_cursor_visible(ctx.editor, *buf);
+		return true;
+	}
+
+	UndoSystem *u = buf->Undo();
+	if (u) {
+		// Start/extend a typed-run batch. Do NOT commit here; commit happens on boundaries
+		// (cursor movement, prompts, undo/redo, etc.) so consecutive InsertText commands coalesce.
+		buf->SetCursor(x, y);
+		u->Begin(UndoType::Insert);
+	}
 	// Apply edits to the underlying PieceTable through Buffer::insert_text,
 	// not directly to the legacy rows_ cache. This ensures Save() persists text.
 	for (int i = 0; i < repeat; ++i) {
 		buf->insert_text(static_cast<int>(y), static_cast<int>(x), std::string_view(ctx.arg));
+		if (u)
+			u->Append(std::string_view(ctx.arg));
 		x += ctx.arg.size();
 	}
 	buf->SetDirty(true);
-	// Record undo for this contiguous insert at the original insertion point
-	if (auto *u = buf->Undo()) {
-		// Position cursor at insertion start for the undo record
-		buf->SetCursor(ins_x, ins_y);
-		u->Begin(UndoType::Insert);
-		for (int i = 0; i < repeat; ++i) {
-			u->Append(std::string_view(ctx.arg));
-		}
-		// Finalize this contiguous insert as a single undoable action
-		u->commit();
-	}
 	buf->SetCursor(x, y);
 	ensure_cursor_visible(ctx.editor, *buf);
 	return true;
@@ -2784,6 +2840,41 @@ cmd_newline(CommandContext &ctx)
 	std::size_t y = buf->Cury();
 	std::size_t x = buf->Curx();
 	int repeat    = ctx.count > 0 ? ctx.count : 1;
+
+	// Visual-line mode: broadcast newline splits across selected lines.
+	if (buf->VisualLineActive()) {
+		const std::size_t sy = buf->VisualLineStartY();
+		const std::size_t ey = buf->VisualLineEndY();
+		const auto &rows     = buf->Rows();
+		if (rows.empty())
+			return true;
+		std::size_t splits_above = 0;
+		if (sy < y)
+			splits_above = std::min(ey, y - 1) - sy + 1;
+
+		// Iterate bottom-up to keep row indices stable while splitting.
+		for (std::size_t yy = ey + 1; yy-- > sy;) {
+			const auto &rows_view = buf->Rows();
+			if (yy >= rows_view.size())
+				continue;
+			std::size_t xx = x;
+			if (xx > rows_view[yy].size())
+				xx = rows_view[yy].size();
+			// First split at the cursor column; subsequent splits create blank lines.
+			buf->split_line(static_cast<int>(yy), static_cast<int>(xx));
+			for (int i = 1; i < repeat; ++i) {
+				buf->split_line(static_cast<int>(yy + i), 0);
+			}
+		}
+
+		buf->SetDirty(true);
+		// Cursor: end up on the final inserted line for the original cursor line.
+		std::size_t new_y = y + static_cast<std::size_t>(repeat);
+		new_y             += splits_above;
+		buf->SetCursor(0, new_y);
+		ensure_cursor_visible(ctx.editor, *buf);
+		return true;
+	}
 	for (int i = 0; i < repeat; ++i) {
 		buf->split_line(static_cast<int>(y), static_cast<int>(x));
 		// Move to start of next line
@@ -2858,6 +2949,50 @@ cmd_backspace(CommandContext &ctx)
 	std::size_t x = buf->Curx();
 	UndoSystem *u = buf->Undo();
 	int repeat    = ctx.count > 0 ? ctx.count : 1;
+
+	// Visual-line mode: broadcast backspace deletes within each selected line.
+	// For now, we do NOT join lines when at column 0 (too ambiguous across multiple lines).
+	if (buf->VisualLineActive()) {
+		const std::size_t sy = buf->VisualLineStartY();
+		const std::size_t ey = buf->VisualLineEndY();
+		const auto &rows     = buf->Rows();
+		std::uint64_t gid    = 0;
+		if (u)
+			gid = u->BeginGroup();
+		(void) gid;
+		std::size_t cx = x;
+		for (std::size_t yy = sy; yy <= ey; ++yy) {
+			if (yy >= rows.size())
+				break;
+			std::size_t xx = x;
+			if (xx > rows[yy].size())
+				xx = rows[yy].size();
+			std::string deleted;
+			for (int i = 0; i < repeat; ++i) {
+				if (xx == 0)
+					break;
+				const auto &rows_view = buf->Rows();
+				if (yy < rows_view.size() && (xx - 1) < rows_view[yy].size())
+					deleted.insert(deleted.begin(), rows_view[yy][xx - 1]);
+				buf->delete_text(static_cast<int>(yy), static_cast<int>(xx - 1), 1);
+				--xx;
+			}
+			if (u && !deleted.empty()) {
+				buf->SetCursor(xx, yy);
+				u->Begin(UndoType::Delete);
+				u->Append(std::string_view(deleted));
+				u->commit();
+			}
+			if (yy == y)
+				cx = xx;
+		}
+		if (u)
+			u->EndGroup();
+		buf->SetDirty(true);
+		buf->SetCursor(cx, y);
+		ensure_cursor_visible(ctx.editor, *buf);
+		return true;
+	}
 	for (int i = 0; i < repeat; ++i) {
 		// Refresh a read-only view of lines for char capture/lengths
 		const auto &rows_view = buf->Rows();
@@ -2910,6 +3045,44 @@ cmd_delete_char(CommandContext &ctx)
 	std::size_t x = buf->Curx();
 	UndoSystem *u = buf->Undo();
 	int repeat    = ctx.count > 0 ? ctx.count : 1;
+
+	// Visual-line mode: broadcast delete-char within each selected line.
+	// For now, we do NOT join lines when at end-of-line.
+	if (buf->VisualLineActive()) {
+		const std::size_t sy = buf->VisualLineStartY();
+		const std::size_t ey = buf->VisualLineEndY();
+		const auto &rows     = buf->Rows();
+		std::uint64_t gid    = 0;
+		if (u)
+			gid = u->BeginGroup();
+		(void) gid;
+		for (std::size_t yy = sy; yy <= ey; ++yy) {
+			if (yy >= rows.size())
+				break;
+			std::size_t xx = x;
+			if (xx > rows[yy].size())
+				xx = rows[yy].size();
+			std::string deleted;
+			for (int i = 0; i < repeat; ++i) {
+				const auto &rows_view = buf->Rows();
+				if (yy >= rows_view.size() || xx >= rows_view[yy].size())
+					break;
+				deleted.push_back(rows_view[yy][xx]);
+				buf->delete_text(static_cast<int>(yy), static_cast<int>(xx), 1);
+			}
+			if (u && !deleted.empty()) {
+				buf->SetCursor(xx, yy);
+				u->Begin(UndoType::Delete);
+				u->Append(std::string_view(deleted));
+				u->commit();
+			}
+		}
+		if (u)
+			u->EndGroup();
+		buf->SetDirty(true);
+		ensure_cursor_visible(ctx.editor, *buf);
+		return true;
+	}
 	for (int i = 0; i < repeat; ++i) {
 		const auto &rows_view = buf->Rows();
 		if (y >= rows_view.size())
@@ -2947,6 +3120,8 @@ cmd_undo(CommandContext &ctx)
 	if (auto *u = buf->Undo()) {
 		// Ensure pending batch is finalized so it can be undone
 		u->commit();
+		int repeat = ctx.count > 0 ? ctx.count : 1;
+		for (int i = 0; i < repeat; ++i)
 			u->undo();
 		// Keep cursor within buffer bounds
 		ensure_cursor_visible(ctx.editor, *buf);
@@ -2966,7 +3141,14 @@ cmd_redo(CommandContext &ctx)
 	if (auto *u = buf->Undo()) {
 		// Finalize any pending batch before redoing
 		u->commit();
+		// With branching undo, a universal-argument count selects an alternate redo branch:
+		//  - no count (or 1): redo the active branch
+		//  - n>1: redo the (n-1)th sibling branch from this point and make it active
+		if (ctx.count > 1) {
+			u->redo(ctx.count - 1);
+		} else {
 			u->redo();
+		}
 		ensure_cursor_visible(ctx.editor, *buf);
 		ctx.editor.SetStatus("Redone");
 		return true;
@@ -3088,9 +3270,68 @@ cmd_yank(CommandContext &ctx)
 	}
 	ensure_at_least_one_line(*buf);
 	int repeat = ctx.count > 0 ? ctx.count : 1;
-	for (int i = 0; i < repeat; ++i) {
-		insert_text_at_cursor(*buf, text);
+	std::string ins;
+	if (repeat == 1) {
+		ins = text;
+	} else {
+		ins.reserve(text.size() * static_cast<std::size_t>(repeat));
+		for (int i = 0; i < repeat; ++i)
+			ins += text;
 	}
+
+	UndoSystem *u = buf->Undo();
+	// Visual-line mode: broadcast yank to beginning-of-line on every affected line.
+	if (buf->VisualLineActive()) {
+		const std::size_t sy = buf->VisualLineStartY();
+		const std::size_t ey = buf->VisualLineEndY();
+		const std::size_t y0 = buf->Cury();
+
+		std::uint64_t gid = 0;
+		if (u)
+			gid = u->BeginGroup();
+		(void) gid;
+
+		// Iterate from bottom to top so insertions don't invalidate remaining line indices.
+		for (std::size_t yy = ey + 1; yy-- > sy;) {
+			buf->SetCursor(0, yy);
+			if (u)
+				u->Begin(UndoType::Paste);
+			insert_text_at_cursor(*buf, ins);
+			if (u) {
+				u->Append(std::string_view(ins));
+				u->commit();
+			}
+		}
+		if (u)
+			u->EndGroup();
+
+		// Keep the point on the primary cursor line (as it was before yank), at the end of the
+		// inserted text for that line.
+		std::size_t nl_count = 0;
+		std::size_t last_nl  = std::string::npos;
+		for (std::size_t i = 0; i < ins.size(); ++i) {
+			if (ins[i] == '\n') {
+				++nl_count;
+				last_nl = i;
+			}
+		}
+		const std::size_t delta_y = nl_count;
+		const std::size_t delta_x = (last_nl == std::string::npos) ? ins.size() : (ins.size() - last_nl - 1);
+		const std::size_t above   = (y0 >= sy) ? (y0 - sy) : 0;
+		buf->SetCursor(delta_x, y0 + delta_y + above * nl_count);
+	} else {
+		if (u)
+			u->Begin(UndoType::Paste);
+		insert_text_at_cursor(*buf, ins);
+		if (u) {
+			u->Append(std::string_view(ins));
+			u->commit();
+		}
+	}
+	// Yank is a paste operation; it should clear the mark/region and any selection highlighting.
+	buf->ClearMark();
+	if (buf->VisualLineActive())
+		buf->VisualLineClear();
 	ensure_cursor_visible(ctx.editor, *buf);
 	// Start a new kill chain only from kill commands; yanking should break it
 	ctx.editor.SetKillChain(false);
@@ -3107,6 +3348,8 @@ cmd_move_file_start(CommandContext &ctx)
 		return false;
 	ensure_at_least_one_line(*buf);
 	buf->SetCursor(0, 0);
+	if (buf->VisualLineActive())
+		buf->VisualLineSetActiveY(buf->Cury());
 	ensure_cursor_visible(ctx.editor, *buf);
 	return true;
 }
@@ -3123,6 +3366,8 @@ cmd_move_file_end(CommandContext &ctx)
 	std::size_t y    = rows.empty() ? 0 : rows.size() - 1;
 	std::size_t x    = rows.empty() ? 0 : rows[y].size();
 	buf->SetCursor(x, y);
+	if (buf->VisualLineActive())
+		buf->VisualLineSetActiveY(buf->Cury());
 	ensure_cursor_visible(ctx.editor, *buf);
 	return true;
 }
@@ -3145,6 +3390,18 @@ cmd_toggle_mark(CommandContext &ctx)
 }
 
 
+static bool
+cmd_visual_line_mode_toggle(CommandContext &ctx)
+{
+	Buffer *buf = ctx.editor.CurrentBuffer();
+	if (!buf)
+		return false;
+	buf->VisualLineToggle();
+	ctx.editor.SetStatus(std::string("Visual line: ") + (buf->VisualLineActive() ? "ON" : "OFF"));
+	return true;
+}
+
+
 static bool
 cmd_jump_to_mark(CommandContext &ctx)
 {
@@ -3306,6 +3563,8 @@ cmd_move_left(CommandContext &ctx)
 		}
 	}
 	buf->SetCursor(x, y);
+	if (buf->VisualLineActive())
+		buf->VisualLineSetActiveY(buf->Cury());
 	ensure_cursor_visible(ctx.editor, *buf);
 	return true;
 }
@@ -3383,6 +3642,8 @@ cmd_move_right(CommandContext &ctx)
 		}
 	}
 	buf->SetCursor(x, y);
+	if (buf->VisualLineActive())
+		buf->VisualLineSetActiveY(buf->Cury());
 	ensure_cursor_visible(ctx.editor, *buf);
 	return true;
 }
@@ -3423,6 +3684,8 @@ cmd_move_up(CommandContext &ctx)
 	if (x > rows[y].size())
 		x = rows[y].size();
 	buf->SetCursor(x, y);
+	if (buf->VisualLineActive())
+		buf->VisualLineSetActiveY(buf->Cury());
 	ensure_cursor_visible(ctx.editor, *buf);
 	return true;
 }
@@ -3464,6 +3727,8 @@ cmd_move_down(CommandContext &ctx)
 	if (x > rows[y].size())
 		x = rows[y].size();
 	buf->SetCursor(x, y);
+	if (buf->VisualLineActive())
+		buf->VisualLineSetActiveY(buf->Cury());
 	ensure_cursor_visible(ctx.editor, *buf);
 	return true;
 }
@@ -4026,6 +4291,29 @@ cmd_reflow_paragraph(CommandContext &ctx)
 		return false;
 	};
 
+	auto is_numbered_line = [&](const std::string &s,
+	                            std::string &indent_out,
+	                            std::string &marker_out,
+	                            std::size_t &after_prefix_idx) -> bool {
+		indent_out    = leading_ws(s);
+		std::size_t i = indent_out.size();
+		if (i >= s.size() || !std::isdigit(static_cast<unsigned char>(s[i])))
+			return false;
+		std::size_t j = i;
+		while (j < s.size() && std::isdigit(static_cast<unsigned char>(s[j])))
+			++j;
+		if (j >= s.size())
+			return false;
+		char delim = s[j];
+		if (!(delim == '.' || delim == ')'))
+			return false;
+		if (j + 1 >= s.size() || s[j + 1] != ' ')
+			return false;
+		marker_out       = s.substr(i, (j - i) + 1); // e.g. "1." or "10)"
+		after_prefix_idx = j + 2; // after delimiter + space
+		return true;
+	};
+
 	auto normalize_spaces = [](const std::string &in) {
 		std::string out;
 		out.reserve(in.size());
@@ -4107,20 +4395,25 @@ cmd_reflow_paragraph(CommandContext &ctx)
 
 	std::vector<std::string> new_lines;
 
-	// Determine if this region looks like a list: any line starting with bullet
-	bool region_has_bullet = false;
+	// Determine if this region looks like a list: any line starting with bullet or number
+	bool region_has_list = false;
 	for (std::size_t i = para_start; i <= para_end; ++i) {
 		std::string s = static_cast<std::string>(rows[i]);
 		std::string indent;
 		char marker;
 		std::size_t idx;
 		if (is_bullet_line(s, indent, marker, idx)) {
-			region_has_bullet = true;
+			region_has_list = true;
+			break;
+		}
+		std::string nmarker;
+		if (is_numbered_line(s, indent, nmarker, idx)) {
+			region_has_list = true;
 			break;
 		}
 	}
 
-	if (region_has_bullet) {
+	if (region_has_list) {
 		// Parse as list items; support hanging indent continuations
 		for (std::size_t i = para_start; i <= para_end; ++i) {
 			std::string s = static_cast<std::string>(rows[i]);
@@ -4148,12 +4441,46 @@ cmd_reflow_paragraph(CommandContext &ctx)
 					if (is_bullet_line(ns, nindent, nmarker, nidx)) {
 						break; // next item
 					}
+					std::string nnmarker;
+					if (is_numbered_line(ns, nindent, nnmarker, nidx)) {
+						break; // next item
+					}
 					// Not a continuation and not a bullet: stop (treat as separate paragraph chunk)
 					break;
 				}
 				content = normalize_spaces(content);
 				wrap_with_prefixes(content, first_prefix, cont_prefix, width, new_lines);
 				i = j - 1; // advance
+			} else {
+				std::string nmarker;
+				if (is_numbered_line(s, indent, nmarker, after_idx)) {
+					std::string first_prefix = indent + nmarker + " ";
+					std::string cont_prefix  = indent + std::string(nmarker.size() + 1, ' ');
+					std::string content      = s.substr(after_idx);
+					// consume continuation lines that are part of this numbered item
+					std::size_t j = i + 1;
+					while (j <= para_end) {
+						std::string ns = static_cast<std::string>(rows[j]);
+						if (starts_with(ns, cont_prefix)) {
+							content += ' ';
+							content += ns.substr(cont_prefix.size());
+							++j;
+							continue;
+						}
+						// stop if next item
+						std::string nindent2;
+						char bmarker;
+						std::size_t nidx;
+						if (is_bullet_line(ns, nindent2, bmarker, nidx))
+							break;
+						std::string nnmarker;
+						if (is_numbered_line(ns, nindent2, nnmarker, nidx))
+							break;
+						break;
+					}
+					content = normalize_spaces(content);
+					wrap_with_prefixes(content, first_prefix, cont_prefix, width, new_lines);
+					i = j - 1;
 				} else {
 					// A non-bullet line within a list region; treat as its own wrapped paragraph preserving its indent
 					std::string base_indent = leading_ws(s);
@@ -4168,6 +4495,10 @@ cmd_reflow_paragraph(CommandContext &ctx)
 						if (is_bullet_line(ns, tmp_indent, tmp_marker, tmp_idx)) {
 							break; // next bullet starts
 						}
+						std::string tmp_nmarker;
+						if (is_numbered_line(ns, tmp_indent, tmp_nmarker, tmp_idx)) {
+							break; // next numbered starts
+						}
 						if (nindent.size() >= base_indent.size()) {
 							content += ' ';
 							content += ns.substr(base_indent.size());
@@ -4181,6 +4512,7 @@ cmd_reflow_paragraph(CommandContext &ctx)
 					i = j - 1;
 				}
 			}
+		}
 	} else {
 		// Normal paragraph: preserve indentation of first line
 		std::string s0  = static_cast<std::string>(rows[para_start]);
@@ -4396,6 +4728,10 @@ InstallDefaultCommands()
 	});
 	CommandRegistry::Register({CommandId::MoveFileEnd, "file-end", "Move to end of file", cmd_move_file_end});
 	CommandRegistry::Register({CommandId::ToggleMark, "toggle-mark", "Toggle mark at cursor", cmd_toggle_mark});
+	CommandRegistry::Register({
+		CommandId::VisualLineModeToggle, "visual-line-toggle", "Toggle visual-line (multicursor) mode",
+		cmd_visual_line_mode_toggle, false, false
+	});
 	CommandRegistry::Register({
 		CommandId::JumpToMark, "jump-to-mark", "Jump to mark (swap mark)", cmd_jump_to_mark
 	});

Command.h

@@ -47,6 +47,7 @@ enum class CommandId {
 	MoveFileStart, // move to beginning of file
 	MoveFileEnd, // move to end of file
 	ToggleMark, // toggle mark at cursor
+	VisualLineModeToggle, // toggle visual-line (multicursor) mode (C-k /)
 	JumpToMark, // jump to mark, set mark to previous cursor
 	KillRegion, // kill region between mark and cursor (to kill ring)
 	CopyRegion, // copy region to kill ring (Alt-w)

Editor.cc

@@ -128,8 +128,8 @@ Editor::AddBuffer(const Buffer &buf)
 	buffers_.push_back(buf);
 	// Attach swap recorder
 	if (swap_) {
-		buffers_.back().SetSwapRecorder(swap_.get());
 		swap_->Attach(&buffers_.back());
+		buffers_.back().SetSwapRecorder(swap_->RecorderFor(&buffers_.back()));
 	}
 	if (buffers_.size() == 1) {
 		curbuf_ = 0;
@@ -143,8 +143,8 @@ Editor::AddBuffer(Buffer &&buf)
 {
 	buffers_.push_back(std::move(buf));
 	if (swap_) {
-		buffers_.back().SetSwapRecorder(swap_.get());
 		swap_->Attach(&buffers_.back());
+		buffers_.back().SetSwapRecorder(swap_->RecorderFor(&buffers_.back()));
 	}
 	if (buffers_.size() == 1) {
 		curbuf_ = 0;
@@ -171,8 +171,8 @@ Editor::OpenFile(const std::string &path, std::string &err)
 				return false;
 			// Ensure swap recorder is attached for this buffer
 			if (swap_) {
-				cur.SetSwapRecorder(swap_.get());
 				swap_->Attach(&cur);
+				cur.SetSwapRecorder(swap_->RecorderFor(&cur));
 				swap_->NotifyFilenameChanged(cur);
 			}
 			// Setup highlighting using registry (extension + shebang)
@@ -207,12 +207,8 @@ Editor::OpenFile(const std::string &path, std::string &err)
 	if (!b.OpenFromFile(path, err)) {
 		return false;
 	}
-	if (swap_) {
-		b.SetSwapRecorder(swap_.get());
-		// path is known, notify
-		swap_->Attach(&b);
-		swap_->NotifyFilenameChanged(b);
-	}
+	// NOTE: swap recorder/attach must happen after the buffer is stored in its
+	// final location (vector) because swap manager keys off Buffer*.
 	// Initialize syntax highlighting by extension + shebang via registry (v2)
 	b.EnsureHighlighter();
 	std::string first = "";
@@ -239,6 +235,9 @@ Editor::OpenFile(const std::string &path, std::string &err)
 	}
 	// Add as a new buffer and switch to it
 	std::size_t idx = AddBuffer(std::move(b));
+	if (swap_) {
+		swap_->NotifyFilenameChanged(buffers_[idx]);
+	}
 	SwitchTo(idx);
 	// Defensive: ensure any active prompt is closed after a successful open
 	CancelPrompt();
@@ -284,6 +283,10 @@ Editor::CloseBuffer(std::size_t index)
 	if (index >= buffers_.size()) {
 		return false;
 	}
+	if (swap_) {
+		swap_->Detach(&buffers_[index]);
+		buffers_[index].SetSwapRecorder(nullptr);
+	}
 	buffers_.erase(buffers_.begin() + static_cast<std::ptrdiff_t>(index));
 	if (buffers_.empty()) {
 		curbuf_ = 0;

ImGuiRenderer.cc

@@ -150,73 +150,84 @@ ImGuiRenderer::Draw(Editor &ed)
 		// Cache current horizontal offset in rendered columns for click handling
 		const std::size_t coloffs_now = buf->Coloffs();
 
-		// Handle mouse click before rendering to avoid dependent on drawn items
-		if (ImGui::IsWindowHovered() && ImGui::IsMouseClicked(ImGuiMouseButton_Left)) {
+		// Mark selection state (mark -> cursor), in source coordinates
+		bool sel_active    = false;
+		std::size_t sel_sy = 0, sel_sx = 0, sel_ey = 0, sel_ex = 0;
+		if (buf->MarkSet()) {
+			sel_sy = buf->MarkCury();
+			sel_sx = buf->MarkCurx();
+			sel_ey = buf->Cury();
+			sel_ex = buf->Curx();
+			if (sel_sy > sel_ey || (sel_sy == sel_ey && sel_sx > sel_ex)) {
+				std::swap(sel_sy, sel_ey);
+				std::swap(sel_sx, sel_ex);
+			}
+			sel_active = !(sel_sy == sel_ey && sel_sx == sel_ex);
+		}
+		// Visual-line selection: full-line highlight range
+		const bool vsel_active    = buf->VisualLineActive();
+		const std::size_t vsel_sy = vsel_active ? buf->VisualLineStartY() : 0;
+		const std::size_t vsel_ey = vsel_active ? buf->VisualLineEndY() : 0;
+
+		static bool mouse_selecting = false;
+		auto mouse_pos_to_buf       = [&]() -> std::pair<std::size_t, std::size_t> {
 			ImVec2 mp = ImGui::GetIO().MousePos;
-			// Compute content-relative position accounting for scroll
-			// mp.y - child_window_pos.y gives us pixels from top of child window
-			// Adding scroll_y gives us pixels from top of content (buffer row 0)
+			// Convert mouse pos to buffer row
 			float content_y = (mp.y - child_window_pos.y) + scroll_y;
 			long by_l       = static_cast<long>(content_y / row_h);
 			if (by_l < 0)
 				by_l = 0;
-
-			// Convert to buffer row
 			std::size_t by = static_cast<std::size_t>(by_l);
-			if (by >= lines.size()) {
-				if (!lines.empty())
-					by = lines.size() - 1;
-				else
-					by = 0;
-			}
+			if (by >= lines.size())
+				by = lines.empty() ? 0 : (lines.size() - 1);
 
-			// Compute click X position relative to left edge of child window (in pixels)
-			// This gives us the visual offset from the start of displayed content
+			// Convert mouse pos to rendered x
 			float visual_x = mp.x - child_window_pos.x;
 			if (visual_x < 0.0f)
 				visual_x = 0.0f;
-
-			// Convert visual pixel offset to rendered column, then add coloffs_now
-			// to get the absolute rendered column in the buffer
 			std::size_t clicked_rx = static_cast<std::size_t>(visual_x / space_w) + coloffs_now;
 
-			// Empty buffer guard: if there are no lines yet, just move to 0:0
-			if (lines.empty()) {
-				Execute(ed, CommandId::MoveCursorTo, std::string("0:0"));
-			} else {
-				// Convert rendered column (clicked_rx) to source column accounting for tabs
+			// Convert rendered column to source column
+			if (lines.empty())
+				return {0, 0};
 			std::string line_clicked = static_cast<std::string>(lines[by]);
 			const std::size_t tabw   = 8;
-
-				// Iterate through source columns, computing rendered position, to find closest match
-				std::size_t rx       = 0; // rendered column position
+			std::size_t rx           = 0;
 			std::size_t best_col     = 0;
 			float best_dist          = std::numeric_limits<float>::infinity();
 			float clicked_rx_f       = static_cast<float>(clicked_rx);
-
 			for (std::size_t i = 0; i <= line_clicked.size(); ++i) {
-					// Check current position
 				float dist = std::fabs(clicked_rx_f - static_cast<float>(rx));
 				if (dist < best_dist) {
 					best_dist = dist;
 					best_col  = i;
 				}
-
-					// Advance to next position if not at end
 				if (i < line_clicked.size()) {
-						if (line_clicked[i] == '\t') {
-							rx += (tabw - (rx % tabw));
-						} else {
-							rx += 1;
-						}
+					rx += (line_clicked[i] == '\t') ? (tabw - (rx % tabw)) : 1;
 				}
 			}
+			return {by, best_col};
+		};
 
-				// Dispatch absolute buffer coordinates (row:col)
+		// Mouse-driven selection: set mark on press, update cursor on drag
+		if (ImGui::IsWindowHovered() && ImGui::IsMouseClicked(ImGuiMouseButton_Left)) {
+			mouse_selecting = true;
+			auto [by, bx]   = mouse_pos_to_buf();
 			char tmp[64];
-				std::snprintf(tmp, sizeof(tmp), "%zu:%zu", by, best_col);
+			std::snprintf(tmp, sizeof(tmp), "%zu:%zu", by, bx);
+			Execute(ed, CommandId::MoveCursorTo, std::string(tmp));
+			if (Buffer *mbuf = const_cast<Buffer *>(buf)) {
+				mbuf->SetMark(bx, by);
+			}
+		}
+		if (mouse_selecting && ImGui::IsWindowHovered() && ImGui::IsMouseDown(ImGuiMouseButton_Left)) {
+			auto [by, bx] = mouse_pos_to_buf();
+			char tmp[64];
+			std::snprintf(tmp, sizeof(tmp), "%zu:%zu", by, bx);
 			Execute(ed, CommandId::MoveCursorTo, std::string(tmp));
 		}
+		if (mouse_selecting && ImGui::IsMouseReleased(ImGuiMouseButton_Left)) {
+			mouse_selecting = false;
 		}
 		for (std::size_t i = rowoffs; i < lines.size(); ++i) {
 			// Capture the screen position before drawing the line
@@ -295,6 +306,71 @@ ImGuiRenderer::Draw(Editor &ed)
 					ImGui::GetWindowDrawList()->AddRectFilled(p0, p1, col);
 				}
 			}
+
+			// Draw selection background (over search highlight; under text)
+			if (sel_active) {
+				bool line_has  = false;
+				std::size_t sx = 0, ex = 0;
+				if (i < sel_sy || i > sel_ey) {
+					line_has = false;
+				} else if (sel_sy == sel_ey) {
+					sx       = sel_sx;
+					ex       = sel_ex;
+					line_has = ex > sx;
+				} else if (i == sel_sy) {
+					sx       = sel_sx;
+					ex       = line.size();
+					line_has = ex > sx;
+				} else if (i == sel_ey) {
+					sx       = 0;
+					ex       = std::min(sel_ex, line.size());
+					line_has = ex > sx;
+				} else {
+					sx       = 0;
+					ex       = line.size();
+					line_has = ex > sx;
+				}
+				if (line_has) {
+					std::size_t rx_start = src_to_rx(sx);
+					std::size_t rx_end   = src_to_rx(ex);
+					if (rx_end > coloffs_now) {
+						std::size_t vx0 = (rx_start > coloffs_now)
+							                  ? (rx_start - coloffs_now)
+							                  : 0;
+						std::size_t vx1 = rx_end - coloffs_now;
+						ImVec2 p0       = ImVec2(line_pos.x + static_cast<float>(vx0) * space_w,
+						                         line_pos.y);
+						ImVec2 p1 = ImVec2(line_pos.x + static_cast<float>(vx1) * space_w,
+						                   line_pos.y + line_h);
+						ImU32 col = ImGui::GetColorU32(ImGuiCol_TextSelectedBg);
+						ImGui::GetWindowDrawList()->AddRectFilled(p0, p1, col);
+					}
+				}
+			}
+			if (vsel_active && i >= vsel_sy && i <= vsel_ey) {
+				// Visual-line (multi-cursor) mode: highlight only the per-line cursor spot.
+				const std::size_t spot_sx  = std::min(buf->Curx(), line.size());
+				const std::size_t rx_start = src_to_rx(spot_sx);
+				std::size_t rx_end         = rx_start;
+				if (spot_sx < line.size()) {
+					rx_end = src_to_rx(spot_sx + 1);
+				} else {
+					// EOL spot: draw a 1-cell highlight just past the last character.
+					rx_end = rx_start + 1;
+				}
+				if (rx_end > coloffs_now) {
+					std::size_t vx0 = (rx_start > coloffs_now)
+						                  ? (rx_start - coloffs_now)
+						                  : 0;
+					std::size_t vx1 = rx_end - coloffs_now;
+					ImVec2 p0       = ImVec2(line_pos.x + static_cast<float>(vx0) * space_w,
+					                         line_pos.y);
+					ImVec2 p1 = ImVec2(line_pos.x + static_cast<float>(vx1) * space_w,
+					                   line_pos.y + line_h);
+					ImU32 col = ImGui::GetColorU32(ImGuiCol_TextSelectedBg);
+					ImGui::GetWindowDrawList()->AddRectFilled(p0, p1, col);
+				}
+			}
 			// Emit entire line to an expanded buffer (tabs -> spaces)
 			for (std::size_t src = 0; src < line.size(); ++src) {
 				char c = line[src];

KKeymap.cc

@@ -17,6 +17,9 @@ KLookupKCommand(const int ascii_key, const bool ctrl, CommandId &out) -> bool
 		case 'd':
 			out = CommandId::KillLine;
 			return true;
+		case 's':
+			out = CommandId::Save;
+			return true;
 		case 'q':
 			out = CommandId::QuitNow;
 			return true;
@@ -42,6 +45,9 @@ KLookupKCommand(const int ascii_key, const bool ctrl, CommandId &out) -> bool
 	case 'a':
 		out = CommandId::MarkAllAndJumpEnd;
 		return true;
+	case ' ': // C-k SPACE
+		out = CommandId::ToggleMark;
+		return true;
 	case 'i':
 		out = CommandId::BufferNew; // C-k i new empty buffer
 		return true;
@@ -114,6 +120,9 @@ KLookupKCommand(const int ascii_key, const bool ctrl, CommandId &out) -> bool
 	case '=':
 		out = CommandId::IndentRegion;
 		return true;
+	case '/':
+		out = CommandId::VisualLineModeToggle;
+		return true;
 	case ';':
 		out = CommandId::CommandPromptStart; // C-k ; : generic command prompt
 		return true;

README.md

@@ -32,7 +32,8 @@ Project Goals
 
 Keybindings
 -----------
-kte maintains ke’s command model while internals evolve. Highlights (subject to refinement):
+kte maintains ke’s command model while internals evolve. Highlights (
+subject to refinement):
 
 - K‑command prefix: `C-k` enters k‑command mode; exit with `ESC` or
   `C-g`.
@@ -52,7 +53,8 @@ See `ke.md` for the canonical ke reference retained for now.
 
 Build and Run
 -------------
-Prerequisites: C++17 compiler, CMake, and ncurses development headers/libs.
+Prerequisites: C++20 compiler, CMake, and ncurses development
+headers/libs.
 
 Dependencies by platform
 ------------------------
@@ -62,30 +64,38 @@ Dependencies by platform
         - `brew install ncurses`
     - Optional GUI (enable with `-DBUILD_GUI=ON`):
         - `brew install sdl2 freetype`
-        - OpenGL is provided by the system framework on macOS; no package needed.
+        - OpenGL is provided by the system framework on macOS; no
+          package needed.
 
 - Debian/Ubuntu
     - Terminal (default):
         - `sudo apt-get install -y libncurses5-dev libncursesw5-dev`
     - Optional GUI (enable with `-DBUILD_GUI=ON`):
-        - `sudo apt-get install -y libsdl2-dev libfreetype6-dev mesa-common-dev`
-        - The `mesa-common-dev` package provides OpenGL headers/libs (`libGL`).
+        -
+        `sudo apt-get install -y libsdl2-dev libfreetype6-dev mesa-common-dev`
+        - The `mesa-common-dev` package provides OpenGL headers/libs (
+          `libGL`).
 
 - NixOS/Nix
     - Terminal (default):
         - Ad-hoc shell: `nix-shell -p cmake gcc ncurses`
     - Optional GUI (enable with `-DBUILD_GUI=ON`):
-        - Ad-hoc shell: `nix-shell -p cmake gcc ncurses SDL2 freetype libGL`
-    - With flakes/devshell (example `flake.nix` inputs not provided): include
-      `ncurses` for TUI, and `SDL2`, `freetype`, `libGL` for GUI in your devShell.
+        - Ad-hoc shell:
+          `nix-shell -p cmake gcc ncurses SDL2 freetype libGL`
+    - With flakes/devshell (example `flake.nix` inputs not provided):
+      include
+      `ncurses` for TUI, and `SDL2`, `freetype`, `libGL` for GUI in your
+      devShell.
 
 Notes
 -----
 
-- The GUI is OFF by default to keep SDL/OpenGL/Freetype optional. Enable it by
+- The GUI is OFF by default to keep SDL/OpenGL/Freetype optional. Enable
+  it by
   configuring with `-DBUILD_GUI=ON` and ensuring the GUI deps above are
   installed for your platform.
-- If you previously configured with GUI ON and want to disable it, reconfigure
+- If you previously configured with GUI ON and want to disable it,
+  reconfigure
   the build directory with `-DBUILD_GUI=OFF`.
 
 Example build:
@@ -113,7 +123,8 @@ built as `kge`) or request the GUI from `kte`:
 GUI build example
 -----------------
 
-To build with the optional GUI (after installing the GUI dependencies listed above):
+To build with the optional GUI (after installing the GUI dependencies
+listed above):
 
 ```
 cmake -S . -B cmake-build-debug -DCMAKE_BUILD_TYPE=Debug -DBUILD_GUI=ON

Swap.cc

@@ -5,6 +5,9 @@
 #include <chrono>
 #include <cstdio>
 #include <cstring>
+#include <ctime>
+#include <cstdlib>
+#include <fstream>
 #include <filesystem>
 #include <fcntl.h>
 #include <unistd.h>
@@ -18,8 +21,51 @@ namespace {
 constexpr std::uint8_t MAGIC[8] = {'K', 'T', 'E', '_', 'S', 'W', 'P', '\0'};
 constexpr std::uint32_t VERSION = 1;
 
+
+static fs::path
+xdg_state_home()
+{
+	if (const char *p = std::getenv("XDG_STATE_HOME")) {
+		if (*p)
+			return fs::path(p);
+	}
+	if (const char *home = std::getenv("HOME")) {
+		if (*home)
+			return fs::path(home) / ".local" / "state";
+	}
+	// Last resort: still provide a stable per-user-ish location.
+	return fs::temp_directory_path() / "kte" / "state";
+}
+
+
+static std::uint64_t
+fnv1a64(std::string_view s)
+{
+	std::uint64_t h = 14695981039346656037ULL;
+	for (unsigned char ch: s) {
+		h ^= (std::uint64_t) ch;
+		h *= 1099511628211ULL;
+	}
+	return h;
+}
+
+
+static std::string
+hex_u64(std::uint64_t v)
+{
+	static const char *kHex = "0123456789abcdef";
+	char out[16];
+	for (int i = 15; i >= 0; --i) {
+		out[i] = kHex[v & 0xFULL];
+		v      >>= 4;
+	}
+	return std::string(out, sizeof(out));
+}
+
+
 // Write all bytes in buf to fd, handling EINTR and partial writes.
-static bool write_full(int fd, const void *buf, size_t len)
+static bool
+write_full(int fd, const void *buf, size_t len)
 {
 	const std::uint8_t *p = static_cast<const std::uint8_t *>(buf);
 	while (len > 0) {
@@ -50,6 +96,8 @@ SwapManager::SwapManager()
 
 SwapManager::~SwapManager()
 {
+	// Best-effort: drain queued records before stopping the writer.
+	Flush();
 	running_.store(false);
 	cv_.notify_all();
 	if (worker_.joinable())
@@ -62,30 +110,108 @@ SwapManager::~SwapManager()
 
 
 void
-SwapManager::Attach(Buffer * /*buf*/)
+SwapManager::Flush(Buffer *buf)
 {
-	// Stage 1: lazy-open on first record; nothing to do here.
+	(void) buf; // stage 1: flushes all buffers
+	std::unique_lock<std::mutex> lk(mtx_);
+	const std::uint64_t target = next_seq_;
+	// Wake the writer in case it's waiting on the interval.
+	cv_.notify_one();
+	cv_.wait(lk, [&] {
+		return queue_.empty() && inflight_ == 0 && last_processed_ >= target;
+	});
 }
 
 
 void
-SwapManager::Detach(Buffer * /*buf*/)
+SwapManager::BufferRecorder::OnInsert(int row, int col, std::string_view bytes)
 {
-	// Stage 1: keep files open until manager destruction; future work can close per-buffer.
+	m_.RecordInsert(buf_, row, col, bytes);
+}
+
+
+void
+SwapManager::BufferRecorder::OnDelete(int row, int col, std::size_t len)
+{
+	m_.RecordDelete(buf_, row, col, len);
+}
+
+
+SwapRecorder *
+SwapManager::RecorderFor(Buffer *buf)
+{
+	if (!buf)
+		return nullptr;
+	std::lock_guard<std::mutex> lg(mtx_);
+	auto it = recorders_.find(buf);
+	if (it != recorders_.end())
+		return it->second.get();
+	// Create on-demand. Recording calls will no-op until Attach() has been called.
+	auto rec          = std::make_unique<BufferRecorder>(*this, *buf);
+	SwapRecorder *ptr = rec.get();
+	recorders_[buf]   = std::move(rec);
+	return ptr;
+}
+
+
+void
+SwapManager::Attach(Buffer *buf)
+{
+	if (!buf)
+		return;
+	std::lock_guard<std::mutex> lg(mtx_);
+	JournalCtx &ctx = journals_[buf];
+	if (ctx.path.empty())
+		ctx.path = ComputeSidecarPath(*buf);
+	// Ensure a recorder exists as well.
+	if (recorders_.find(buf) == recorders_.end()) {
+		recorders_[buf] = std::make_unique<BufferRecorder>(*this, *buf);
+	}
+}
+
+
+void
+SwapManager::Detach(Buffer *buf)
+{
+	if (!buf)
+		return;
+	{
+		std::lock_guard<std::mutex> lg(mtx_);
+		auto it = journals_.find(buf);
+		if (it != journals_.end()) {
+			it->second.suspended = true;
+		}
+	}
+	Flush(buf);
+	std::lock_guard<std::mutex> lg(mtx_);
+	auto it = journals_.find(buf);
+	if (it != journals_.end()) {
+		close_ctx(it->second);
+		journals_.erase(it);
+	}
+	recorders_.erase(buf);
 }
 
 
 void
 SwapManager::NotifyFilenameChanged(Buffer &buf)
 {
+	{
+		std::lock_guard<std::mutex> lg(mtx_);
+		auto it = journals_.find(&buf);
+		if (it == journals_.end())
+			return;
+		it->second.suspended = true;
+	}
+	Flush(&buf);
 	std::lock_guard<std::mutex> lg(mtx_);
 	auto it = journals_.find(&buf);
 	if (it == journals_.end())
 		return;
 	JournalCtx &ctx = it->second;
-	// Close existing file handle, update path; lazily reopen on next write
 	close_ctx(ctx);
 	ctx.path      = ComputeSidecarPath(buf);
+	ctx.suspended = false;
 }
 
 
@@ -93,47 +219,100 @@ void
 SwapManager::SetSuspended(Buffer &buf, bool on)
 {
 	std::lock_guard<std::mutex> lg(mtx_);
-	auto path = ComputeSidecarPath(buf);
-	// Create/update context for this buffer
-	JournalCtx &ctx = journals_[&buf];
-	ctx.path        = path;
-	ctx.suspended   = on;
+	auto it = journals_.find(&buf);
+	if (it == journals_.end())
+		return;
+	it->second.suspended = on;
 }
 
 
 SwapManager::SuspendGuard::SuspendGuard(SwapManager &m, Buffer *b)
 	: m_(m), buf_(b), prev_(false)
 {
-	// Suspend recording while guard is alive
-	if (buf_)
-		m_.SetSuspended(*buf_, true);
+	if (!buf_)
+		return;
+	{
+		std::lock_guard<std::mutex> lg(m_.mtx_);
+		auto it = m_.journals_.find(buf_);
+		if (it != m_.journals_.end()) {
+			prev_                = it->second.suspended;
+			it->second.suspended = true;
+		}
+	}
 }
 
 
 SwapManager::SuspendGuard::~SuspendGuard()
 {
-	if (buf_)
-		m_.SetSuspended(*buf_, false);
+	if (!buf_)
+		return;
+	std::lock_guard<std::mutex> lg(m_.mtx_);
+	auto it = m_.journals_.find(buf_);
+	if (it != m_.journals_.end()) {
+		it->second.suspended = prev_;
+	}
 }
 
 
 std::string
 SwapManager::ComputeSidecarPath(const Buffer &buf)
 {
-	if (buf.IsFileBacked() || !buf.Filename().empty()) {
-		fs::path p(buf.Filename());
-		fs::path dir     = p.parent_path();
-		std::string base = p.filename().string();
-		std::string side = "." + base + ".kte.swp";
-		return (dir / side).string();
+	// Always place swap under an XDG home-appropriate state directory.
+	// This avoids cluttering working directories and prevents stomping on
+	// swap files when multiple different paths share the same basename.
+	fs::path root = xdg_state_home() / "kte" / "swap";
+
+	auto encode_path = [](std::string s) -> std::string {
+		// Turn an absolute path like "/home/kyle/tmp/test.txt" into
+		// "home!kyle!tmp!test.txt" so swap files are human-identifiable.
+		//
+		// Notes:
+		//  - We strip a single leading path separator so absolute paths don't start with '!'.
+		//  - We replace both '/' and '\\' with '!'.
+		//  - We leave other characters as-is (spaces are OK on POSIX).
+		if (!s.empty() && (s[0] == '/' || s[0] == '\\'))
+			s.erase(0, 1);
+		for (char &ch: s) {
+			if (ch == '/' || ch == '\\')
+				ch = '!';
 		}
-	// unnamed: $TMPDIR/kte/unnamed-<ptr>.kte.swp (best-effort)
-	const char *tmp = std::getenv("TMPDIR");
-	fs::path t      = tmp ? fs::path(tmp) : fs::temp_directory_path();
-	fs::path d      = t / "kte";
-	char bufptr[32];
-	std::snprintf(bufptr, sizeof(bufptr), "%p", (const void *) &buf);
-	return (d / (std::string("unnamed-") + bufptr + ".kte.swp")).string();
+		return s;
+	};
+
+	if (!buf.Filename().empty()) {
+		fs::path p(buf.Filename());
+		std::string key;
+		try {
+			key = fs::weakly_canonical(p).string();
+		} catch (...) {
+			try {
+				key = fs::absolute(p).string();
+			} catch (...) {
+				key = buf.Filename();
+			}
+		}
+		std::string encoded = encode_path(key);
+		if (!encoded.empty()) {
+			std::string name = encoded + ".swp";
+			// Avoid filesystem/path length issues; fall back to hashed naming.
+			// NAME_MAX is often 255 on POSIX, but keep extra headroom.
+			if (name.size() <= 200) {
+				return (root / name).string();
+			}
+		}
+
+		// Fallback: stable, shorter name based on basename + hash.
+		std::string base       = p.filename().string();
+		const std::string name = base + "." + hex_u64(fnv1a64(key)) + ".swp";
+		return (root / name).string();
+	}
+
+	// Unnamed buffers: unique within the process.
+	static std::atomic<std::uint64_t> ctr{0};
+	const std::uint64_t n  = ++ctr;
+	const int pid          = (int) ::getpid();
+	const std::string name = "unnamed-" + std::to_string(pid) + "-" + std::to_string(n) + ".swp";
+	return (root / name).string();
 }
 
 
@@ -163,53 +342,69 @@ SwapManager::ensure_parent_dir(const std::string &path)
 
 
 bool
-SwapManager::write_header(JournalCtx &ctx)
+SwapManager::write_header(int fd)
 {
-	if (ctx.fd < 0)
+	if (fd < 0)
 		return false;
-	// Write a simple 64-byte header
+	// Fixed 64-byte header (v1)
+	// [magic 8][version u32][flags u32][created_time u64][reserved/padding]
 	std::uint8_t hdr[64];
 	std::memset(hdr, 0, sizeof(hdr));
 	std::memcpy(hdr, MAGIC, 8);
-	std::uint32_t ver = VERSION;
-	std::memcpy(hdr + 8, &ver, sizeof(ver));
+	// version (little-endian)
+	hdr[8]  = static_cast<std::uint8_t>(VERSION & 0xFFu);
+	hdr[9]  = static_cast<std::uint8_t>((VERSION >> 8) & 0xFFu);
+	hdr[10] = static_cast<std::uint8_t>((VERSION >> 16) & 0xFFu);
+	hdr[11] = static_cast<std::uint8_t>((VERSION >> 24) & 0xFFu);
+	// flags = 0
+	// created_time (unix seconds; little-endian)
 	std::uint64_t ts = static_cast<std::uint64_t>(std::time(nullptr));
-	std::memcpy(hdr + 16, &ts, sizeof(ts));
-	ssize_t w = ::write(ctx.fd, hdr, sizeof(hdr));
-	return (w == (ssize_t) sizeof(hdr));
+	put_le64(hdr + 16, ts);
+	return write_full(fd, hdr, sizeof(hdr));
 }
 
 
 bool
-SwapManager::open_ctx(JournalCtx &ctx)
+SwapManager::open_ctx(JournalCtx &ctx, const std::string &path)
 {
 	if (ctx.fd >= 0)
 		return true;
-	if (!ensure_parent_dir(ctx.path))
+	if (!ensure_parent_dir(path))
 		return false;
-	// Create or open with 0600 perms
-	int fd = ::open(ctx.path.c_str(), O_CREAT | O_RDWR, 0600);
+	int flags = O_CREAT | O_WRONLY | O_APPEND;
+#ifdef O_CLOEXEC
+	flags |= O_CLOEXEC;
+#endif
+	int fd = ::open(path.c_str(), flags, 0600);
 	if (fd < 0)
 		return false;
-	// Detect if file is new/empty to write header
+	// Ensure permissions even if file already existed.
+	(void) ::fchmod(fd, 0600);
 	struct stat st{};
 	if (fstat(fd, &st) != 0) {
 		::close(fd);
 		return false;
 	}
-	ctx.fd   = fd;
-	ctx.file = fdopen(fd, "ab");
-	if (!ctx.file) {
+	// If an existing file is too small to contain the fixed header, truncate
+	// and restart.
+	if (st.st_size > 0 && st.st_size < 64) {
 		::close(fd);
-		ctx.fd = -1;
+		int tflags = O_CREAT | O_WRONLY | O_TRUNC | O_APPEND;
+#ifdef O_CLOEXEC
+		tflags |= O_CLOEXEC;
+#endif
+		fd = ::open(path.c_str(), tflags, 0600);
+		if (fd < 0)
 			return false;
+		(void) ::fchmod(fd, 0600);
+		st.st_size = 0;
 	}
+	ctx.fd   = fd;
+	ctx.path = path;
 	if (st.st_size == 0) {
-		ctx.header_ok = write_header(ctx);
+		ctx.header_ok = write_header(fd);
 	} else {
-		ctx.header_ok = true; // trust existing file for stage 1
-		// Seek to end to append
-		::lseek(ctx.fd, 0, SEEK_END);
+		ctx.header_ok = true; // stage 1: trust existing header
 	}
 	return ctx.header_ok;
 }
@@ -218,16 +413,12 @@ SwapManager::open_ctx(JournalCtx &ctx)
 void
 SwapManager::close_ctx(JournalCtx &ctx)
 {
-	if (ctx.file) {
-		std::fflush((FILE *) ctx.file);
-		::fsync(ctx.fd);
-		std::fclose((FILE *) ctx.file);
-		ctx.file = nullptr;
-	}
 	if (ctx.fd >= 0) {
+		(void) ::fsync(ctx.fd);
 		::close(ctx.fd);
 		ctx.fd = -1;
 	}
+	ctx.header_ok = false;
 }
 
 
@@ -253,22 +444,35 @@ SwapManager::crc32(const std::uint8_t *data, std::size_t len, std::uint32_t seed
 
 
 void
-SwapManager::put_varu64(std::vector<std::uint8_t> &out, std::uint64_t v)
+SwapManager::put_le32(std::vector<std::uint8_t> &out, std::uint32_t v)
 {
-	while (v >= 0x80) {
-		out.push_back(static_cast<std::uint8_t>(v) | 0x80);
-		v >>= 7;
-	}
-	out.push_back(static_cast<std::uint8_t>(v));
+	out.push_back(static_cast<std::uint8_t>(v & 0xFFu));
+	out.push_back(static_cast<std::uint8_t>((v >> 8) & 0xFFu));
+	out.push_back(static_cast<std::uint8_t>((v >> 16) & 0xFFu));
+	out.push_back(static_cast<std::uint8_t>((v >> 24) & 0xFFu));
 }
 
 
 void
-SwapManager::put_u24(std::uint8_t dst[3], std::uint32_t v)
+SwapManager::put_le64(std::uint8_t *dst, std::uint64_t v)
 {
-	dst[0] = static_cast<std::uint8_t>((v >> 16) & 0xFF);
-	dst[1] = static_cast<std::uint8_t>((v >> 8) & 0xFF);
-	dst[2] = static_cast<std::uint8_t>(v & 0xFF);
+	dst[0] = static_cast<std::uint8_t>(v & 0xFFu);
+	dst[1] = static_cast<std::uint8_t>((v >> 8) & 0xFFu);
+	dst[2] = static_cast<std::uint8_t>((v >> 16) & 0xFFu);
+	dst[3] = static_cast<std::uint8_t>((v >> 24) & 0xFFu);
+	dst[4] = static_cast<std::uint8_t>((v >> 32) & 0xFFu);
+	dst[5] = static_cast<std::uint8_t>((v >> 40) & 0xFFu);
+	dst[6] = static_cast<std::uint8_t>((v >> 48) & 0xFFu);
+	dst[7] = static_cast<std::uint8_t>((v >> 56) & 0xFFu);
+}
+
+
+void
+SwapManager::put_u24_le(std::uint8_t dst[3], std::uint32_t v)
+{
+	dst[0] = static_cast<std::uint8_t>(v & 0xFFu);
+	dst[1] = static_cast<std::uint8_t>((v >> 8) & 0xFFu);
+	dst[2] = static_cast<std::uint8_t>((v >> 16) & 0xFFu);
 }
 
 
@@ -277,6 +481,7 @@ SwapManager::enqueue(Pending &&p)
 {
 	{
 		std::lock_guard<std::mutex> lg(mtx_);
+		p.seq = ++next_seq_;
 		queue_.emplace_back(std::move(p));
 	}
 	cv_.notify_one();
@@ -288,16 +493,20 @@ SwapManager::RecordInsert(Buffer &buf, int row, int col, std::string_view text)
 {
 	{
 		std::lock_guard<std::mutex> lg(mtx_);
-		if (journals_[&buf].suspended)
+		auto it = journals_.find(&buf);
+		if (it == journals_.end() || it->second.suspended)
 			return;
 	}
 	Pending p;
 	p.buf  = &buf;
 	p.type = SwapRecType::INS;
-	// payload: varint row, varint col, varint len, bytes
-	put_varu64(p.payload, static_cast<std::uint64_t>(std::max(0, row)));
-	put_varu64(p.payload, static_cast<std::uint64_t>(std::max(0, col)));
-	put_varu64(p.payload, static_cast<std::uint64_t>(text.size()));
+	// payload v1: [encver u8=1][row u32][col u32][nbytes u32][bytes]
+	if (text.size() > 0xFFFFFFFFu)
+		return;
+	p.payload.push_back(1);
+	put_le32(p.payload, static_cast<std::uint32_t>(std::max(0, row)));
+	put_le32(p.payload, static_cast<std::uint32_t>(std::max(0, col)));
+	put_le32(p.payload, static_cast<std::uint32_t>(text.size()));
 	p.payload.insert(p.payload.end(), reinterpret_cast<const std::uint8_t *>(text.data()),
 	                 reinterpret_cast<const std::uint8_t *>(text.data()) + text.size());
 	enqueue(std::move(p));
@@ -309,15 +518,20 @@ SwapManager::RecordDelete(Buffer &buf, int row, int col, std::size_t len)
 {
 	{
 		std::lock_guard<std::mutex> lg(mtx_);
-		if (journals_[&buf].suspended)
+		auto it = journals_.find(&buf);
+		if (it == journals_.end() || it->second.suspended)
 			return;
 	}
+	if (len > 0xFFFFFFFFu)
+		return;
 	Pending p;
 	p.buf  = &buf;
 	p.type = SwapRecType::DEL;
-	put_varu64(p.payload, static_cast<std::uint64_t>(std::max(0, row)));
-	put_varu64(p.payload, static_cast<std::uint64_t>(std::max(0, col)));
-	put_varu64(p.payload, static_cast<std::uint64_t>(len));
+	// payload v1: [encver u8=1][row u32][col u32][len u32]
+	p.payload.push_back(1);
+	put_le32(p.payload, static_cast<std::uint32_t>(std::max(0, row)));
+	put_le32(p.payload, static_cast<std::uint32_t>(std::max(0, col)));
+	put_le32(p.payload, static_cast<std::uint32_t>(len));
 	enqueue(std::move(p));
 }
 
@@ -327,14 +541,17 @@ SwapManager::RecordSplit(Buffer &buf, int row, int col)
 {
 	{
 		std::lock_guard<std::mutex> lg(mtx_);
-		if (journals_[&buf].suspended)
+		auto it = journals_.find(&buf);
+		if (it == journals_.end() || it->second.suspended)
 			return;
 	}
 	Pending p;
 	p.buf  = &buf;
 	p.type = SwapRecType::SPLIT;
-	put_varu64(p.payload, static_cast<std::uint64_t>(std::max(0, row)));
-	put_varu64(p.payload, static_cast<std::uint64_t>(std::max(0, col)));
+	// payload v1: [encver u8=1][row u32][col u32]
+	p.payload.push_back(1);
+	put_le32(p.payload, static_cast<std::uint32_t>(std::max(0, row)));
+	put_le32(p.payload, static_cast<std::uint32_t>(std::max(0, col)));
 	enqueue(std::move(p));
 }
 
@@ -344,13 +561,16 @@ SwapManager::RecordJoin(Buffer &buf, int row)
 {
 	{
 		std::lock_guard<std::mutex> lg(mtx_);
-		if (journals_[&buf].suspended)
+		auto it = journals_.find(&buf);
+		if (it == journals_.end() || it->second.suspended)
 			return;
 	}
 	Pending p;
 	p.buf  = &buf;
 	p.type = SwapRecType::JOIN;
-	put_varu64(p.payload, static_cast<std::uint64_t>(std::max(0, row)));
+	// payload v1: [encver u8=1][row u32]
+	p.payload.push_back(1);
+	put_le32(p.payload, static_cast<std::uint32_t>(std::max(0, row)));
 	enqueue(std::move(p));
 }
 
@@ -358,59 +578,91 @@ SwapManager::RecordJoin(Buffer &buf, int row)
 void
 SwapManager::writer_loop()
 {
-	while (running_.load()) {
+	for (;;) {
 		std::vector<Pending> batch;
 		{
 			std::unique_lock<std::mutex> lk(mtx_);
 			if (queue_.empty()) {
+				if (!running_.load())
+					break;
 				cv_.wait_for(lk, std::chrono::milliseconds(cfg_.flush_interval_ms));
 			}
 			if (!queue_.empty()) {
 				batch.swap(queue_);
+				inflight_ += batch.size();
 			}
 		}
 		if (batch.empty())
 			continue;
 
-		// Group by buffer path to minimize fsyncs
 		for (const Pending &p: batch) {
 			process_one(p);
+			{
+				std::lock_guard<std::mutex> lg(mtx_);
+				if (p.seq > last_processed_)
+					last_processed_ = p.seq;
+				if (inflight_ > 0)
+					--inflight_;
+			}
+			cv_.notify_all();
 		}
 
-		// Throttled fsync: best-effort
-		// Iterate unique contexts and fsync if needed
-		// For stage 1, fsync all once per interval
+		// Throttled fsync: best-effort (grouped)
+		std::vector<int> to_sync;
 		std::uint64_t now = now_ns();
+		{
+			std::lock_guard<std::mutex> lg(mtx_);
 			for (auto &kv: journals_) {
 				JournalCtx &ctx = kv.second;
 				if (ctx.fd >= 0) {
-				if (ctx.last_fsync_ns == 0 || (now - ctx.last_fsync_ns) / 1000000ULL >= cfg_.
-				    fsync_interval_ms) {
-					::fsync(ctx.fd);
+					if (ctx.last_fsync_ns == 0 || (now - ctx.last_fsync_ns) / 1000000ULL >=
+					    cfg_.fsync_interval_ms) {
 						ctx.last_fsync_ns = now;
+						to_sync.push_back(ctx.fd);
 					}
 				}
 			}
 		}
+		for (int fd: to_sync) {
+			(void) ::fsync(fd);
+		}
+	}
+	// Wake any waiters.
+	cv_.notify_all();
 }
 
 
 void
 SwapManager::process_one(const Pending &p)
 {
+	if (!p.buf)
+		return;
 	Buffer &buf = *p.buf;
-	// Resolve context by path derived from buffer
-	std::string path = ComputeSidecarPath(buf);
-	// Get or create context keyed by this buffer pointer (stage 1 simplification)
-	JournalCtx &ctx = journals_[p.buf];
-	if (ctx.path.empty())
-		ctx.path = path;
-	if (!open_ctx(ctx))
+
+	JournalCtx *ctxp = nullptr;
+	std::string path;
+	{
+		std::lock_guard<std::mutex> lg(mtx_);
+		auto it = journals_.find(p.buf);
+		if (it == journals_.end())
+			return;
+		if (it->second.suspended)
+			return;
+		if (it->second.path.empty())
+			it->second.path = ComputeSidecarPath(buf);
+		path = it->second.path;
+		ctxp = &it->second;
+	}
+	if (!ctxp)
+		return;
+	if (!open_ctx(*ctxp, path))
+		return;
+	if (p.payload.size() > 0xFFFFFFu)
 		return;
 
 	// Build record: [type u8][len u24][payload][crc32 u32]
 	std::uint8_t len3[3];
-	put_u24(len3, static_cast<std::uint32_t>(p.payload.size()));
+	put_u24_le(len3, static_cast<std::uint32_t>(p.payload.size()));
 
 	std::uint8_t head[4];
 	head[0] = static_cast<std::uint8_t>(p.type);
@@ -422,13 +674,170 @@ SwapManager::process_one(const Pending &p)
 	c               = crc32(head, sizeof(head), c);
 	if (!p.payload.empty())
 		c = crc32(p.payload.data(), p.payload.size(), c);
+	std::uint8_t crcbytes[4];
+	crcbytes[0] = static_cast<std::uint8_t>(c & 0xFFu);
+	crcbytes[1] = static_cast<std::uint8_t>((c >> 8) & 0xFFu);
+	crcbytes[2] = static_cast<std::uint8_t>((c >> 16) & 0xFFu);
+	crcbytes[3] = static_cast<std::uint8_t>((c >> 24) & 0xFFu);
 
 	// Write (handle partial writes and check results)
-    bool ok = write_full(ctx.fd, head, sizeof(head));
+	bool ok = write_full(ctxp->fd, head, sizeof(head));
 	if (ok && !p.payload.empty())
-        ok = write_full(ctx.fd, p.payload.data(), p.payload.size());
+		ok = write_full(ctxp->fd, p.payload.data(), p.payload.size());
 	if (ok)
-        ok = write_full(ctx.fd, &c, sizeof(c));
+		ok = write_full(ctxp->fd, crcbytes, sizeof(crcbytes));
 	(void) ok; // stage 1: best-effort; future work could mark ctx error state
 }
+
+
+static bool
+read_exact(std::ifstream &in, void *dst, std::size_t n)
+{
+	in.read(static_cast<char *>(dst), static_cast<std::streamsize>(n));
+	return in.good() && static_cast<std::size_t>(in.gcount()) == n;
+}
+
+
+static std::uint32_t
+read_le32(const std::uint8_t b[4])
+{
+	return (std::uint32_t) b[0] | ((std::uint32_t) b[1] << 8) | ((std::uint32_t) b[2] << 16) | (
+		       (std::uint32_t) b[3] << 24);
+}
+
+
+static bool
+parse_u32_le(const std::vector<std::uint8_t> &p, std::size_t &off, std::uint32_t &out)
+{
+	if (off + 4 > p.size())
+		return false;
+	out = (std::uint32_t) p[off] | ((std::uint32_t) p[off + 1] << 8) | ((std::uint32_t) p[off + 2] << 16) |
+	      ((std::uint32_t) p[off + 3] << 24);
+	off += 4;
+	return true;
+}
+
+
+bool
+SwapManager::ReplayFile(Buffer &buf, const std::string &swap_path, std::string &err)
+{
+	err.clear();
+	std::ifstream in(swap_path, std::ios::binary);
+	if (!in) {
+		err = "Failed to open swap file for replay: " + swap_path;
+		return false;
+	}
+
+	std::uint8_t hdr[64];
+	if (!read_exact(in, hdr, sizeof(hdr))) {
+		err = "Swap file truncated (header): " + swap_path;
+		return false;
+	}
+	if (std::memcmp(hdr, MAGIC, 8) != 0) {
+		err = "Swap file has bad magic: " + swap_path;
+		return false;
+	}
+	const std::uint32_t ver = read_le32(hdr + 8);
+	if (ver != VERSION) {
+		err = "Unsupported swap version: " + std::to_string(ver);
+		return false;
+	}
+
+	for (;;) {
+		std::uint8_t head[4];
+		in.read(reinterpret_cast<char *>(head), sizeof(head));
+		const std::size_t got_head = static_cast<std::size_t>(in.gcount());
+		if (got_head == 0 && in.eof()) {
+			return true; // clean EOF
+		}
+		if (got_head != sizeof(head)) {
+			err = "Swap file truncated (record header): " + swap_path;
+			return false;
+		}
+
+		const SwapRecType type = static_cast<SwapRecType>(head[0]);
+		const std::size_t len  = (std::size_t) head[1] | ((std::size_t) head[2] << 8) | (
+			                         (std::size_t) head[3] << 16);
+		std::vector<std::uint8_t> payload;
+		payload.resize(len);
+		if (len > 0 && !read_exact(in, payload.data(), len)) {
+			err = "Swap file truncated (payload): " + swap_path;
+			return false;
+		}
+		std::uint8_t crcbytes[4];
+		if (!read_exact(in, crcbytes, sizeof(crcbytes))) {
+			err = "Swap file truncated (crc): " + swap_path;
+			return false;
+		}
+		const std::uint32_t want_crc = read_le32(crcbytes);
+		std::uint32_t got_crc        = 0;
+		got_crc                      = crc32(head, sizeof(head), got_crc);
+		if (!payload.empty())
+			got_crc = crc32(payload.data(), payload.size(), got_crc);
+		if (got_crc != want_crc) {
+			err = "Swap file CRC mismatch: " + swap_path;
+			return false;
+		}
+
+		// Apply record
+		std::size_t off = 0;
+		if (payload.empty()) {
+			err = "Swap record missing payload";
+			return false;
+		}
+		const std::uint8_t encver = payload[off++];
+		if (encver != 1) {
+			err = "Unsupported swap payload encoding";
+			return false;
+		}
+		switch (type) {
+		case SwapRecType::INS: {
+			std::uint32_t row = 0, col = 0, nbytes = 0;
+			if (!parse_u32_le(payload, off, row) || !parse_u32_le(payload, off, col) || !parse_u32_le(
+				    payload, off, nbytes)) {
+				err = "Malformed INS payload";
+				return false;
+			}
+			if (off + nbytes > payload.size()) {
+				err = "Truncated INS payload bytes";
+				return false;
+			}
+			buf.insert_text((int) row, (int) col,
+			                std::string_view(reinterpret_cast<const char *>(payload.data() + off), nbytes));
+			break;
+		}
+		case SwapRecType::DEL: {
+			std::uint32_t row = 0, col = 0, dlen = 0;
+			if (!parse_u32_le(payload, off, row) || !parse_u32_le(payload, off, col) || !parse_u32_le(
+				    payload, off, dlen)) {
+				err = "Malformed DEL payload";
+				return false;
+			}
+			buf.delete_text((int) row, (int) col, (std::size_t) dlen);
+			break;
+		}
+		case SwapRecType::SPLIT: {
+			std::uint32_t row = 0, col = 0;
+			if (!parse_u32_le(payload, off, row) || !parse_u32_le(payload, off, col)) {
+				err = "Malformed SPLIT payload";
+				return false;
+			}
+			buf.split_line((int) row, (int) col);
+			break;
+		}
+		case SwapRecType::JOIN: {
+			std::uint32_t row = 0;
+			if (!parse_u32_le(payload, off, row)) {
+				err = "Malformed JOIN payload";
+				return false;
+			}
+			buf.join_lines((int) row);
+			break;
+		}
+		default:
+			// Ignore unknown types for forward-compat in stage 1
+			break;
+		}
+	}
+}
 } // namespace kte

Swap.h

@@ -7,11 +7,14 @@
 #include <string_view>
 #include <vector>
 #include <unordered_map>
+#include <memory>
 #include <mutex>
 #include <condition_variable>
 #include <thread>
 #include <atomic>
 
+#include "SwapRecorder.h"
+
 class Buffer;
 
 namespace kte {
@@ -31,30 +34,12 @@ struct SwapConfig {
 	unsigned fsync_interval_ms{1000}; // at most once per second
 };
 
-// Lightweight interface that Buffer can call without depending on full manager impl
-class SwapRecorder {
-public:
-	virtual ~SwapRecorder() = default;
-
-	virtual void RecordInsert(Buffer &buf, int row, int col, std::string_view text) = 0;
-
-	virtual void RecordDelete(Buffer &buf, int row, int col, std::size_t len) = 0;
-
-	virtual void RecordSplit(Buffer &buf, int row, int col) = 0;
-
-	virtual void RecordJoin(Buffer &buf, int row) = 0;
-
-	virtual void NotifyFilenameChanged(Buffer &buf) = 0;
-
-	virtual void SetSuspended(Buffer &buf, bool on) = 0;
-};
-
 // SwapManager manages sidecar swap files and a single background writer thread.
-class SwapManager final : public SwapRecorder {
+class SwapManager final {
 public:
 	SwapManager();
 
-	~SwapManager() override;
+	~SwapManager();
 
 	// Attach a buffer to begin journaling. Safe to call multiple times; idempotent.
 	void Attach(Buffer *buf);
@@ -62,17 +47,34 @@ public:
 	// Detach and close journal.
 	void Detach(Buffer *buf);
 
-	// SwapRecorder: Notify that the buffer's filename changed (e.g., SaveAs)
-	void NotifyFilenameChanged(Buffer &buf) override;
+	// Block until all currently queued records have been written.
+	// If buf is non-null, flushes all records (stage 1) but is primarily intended
+	// for tests and shutdown.
+	void Flush(Buffer *buf = nullptr);
 
-	// SwapRecorder
-	void RecordInsert(Buffer &buf, int row, int col, std::string_view text) override;
+	// Obtain a per-buffer recorder adapter that emits records for that buffer.
+	// The returned pointer is owned by the SwapManager and remains valid until
+	// Detach(buf) or SwapManager destruction.
+	SwapRecorder *RecorderFor(Buffer *buf);
 
-	void RecordDelete(Buffer &buf, int row, int col, std::size_t len) override;
+	// Notify that the buffer's filename changed (e.g., SaveAs)
+	void NotifyFilenameChanged(Buffer &buf);
 
-	void RecordSplit(Buffer &buf, int row, int col) override;
+	// Replay a swap journal into an already-open buffer.
+	// On success, the buffer content reflects all valid journal records.
+	// On failure (corrupt/truncated/invalid), the buffer is left in whatever
+	// state results from applying records up to the failure point; callers should
+	// treat this as a recovery failure and surface `err`.
+	static bool ReplayFile(Buffer &buf, const std::string &swap_path, std::string &err);
 
-	void RecordJoin(Buffer &buf, int row) override;
+	// Test-only hook to keep swap path logic centralized.
+	// (Avoid duplicating naming rules in unit tests.)
+#ifdef KTE_TESTS
+	static std::string ComputeSwapPathForTests(const Buffer &buf)
+	{
+		return ComputeSidecarPath(buf);
+	}
+#endif
 
 	// RAII guard to suspend recording for internal operations
 	class SuspendGuard {
@@ -88,12 +90,32 @@ public:
 	};
 
 	// Per-buffer toggle
-	void SetSuspended(Buffer &buf, bool on) override;
+	void SetSuspended(Buffer &buf, bool on);
 
 private:
+	class BufferRecorder final : public SwapRecorder {
+	public:
+		BufferRecorder(SwapManager &m, Buffer &b) : m_(m), buf_(b) {}
+
+		void OnInsert(int row, int col, std::string_view bytes) override;
+
+		void OnDelete(int row, int col, std::size_t len) override;
+
+	private:
+		SwapManager &m_;
+		Buffer &buf_;
+	};
+
+	void RecordInsert(Buffer &buf, int row, int col, std::string_view text);
+
+	void RecordDelete(Buffer &buf, int row, int col, std::size_t len);
+
+	void RecordSplit(Buffer &buf, int row, int col);
+
+	void RecordJoin(Buffer &buf, int row);
+
 	struct JournalCtx {
 		std::string path;
-		void *file{nullptr}; // FILE*
 		int fd{-1};
 		bool header_ok{false};
 		bool suspended{false};
@@ -106,6 +128,7 @@ private:
 		SwapRecType type{SwapRecType::INS};
 		std::vector<std::uint8_t> payload; // framed payload only
 		bool urgent_flush{false};
+		std::uint64_t seq{0};
 	};
 
 	// Helpers
@@ -115,17 +138,19 @@ private:
 
 	static bool ensure_parent_dir(const std::string &path);
 
-	static bool write_header(JournalCtx &ctx);
+	static bool write_header(int fd);
 
-	static bool open_ctx(JournalCtx &ctx);
+	static bool open_ctx(JournalCtx &ctx, const std::string &path);
 
 	static void close_ctx(JournalCtx &ctx);
 
 	static std::uint32_t crc32(const std::uint8_t *data, std::size_t len, std::uint32_t seed = 0);
 
-	static void put_varu64(std::vector<std::uint8_t> &out, std::uint64_t v);
+	static void put_le32(std::vector<std::uint8_t> &out, std::uint32_t v);
 
-	static void put_u24(std::uint8_t dst[3], std::uint32_t v);
+	static void put_le64(std::uint8_t dst[8], std::uint64_t v);
+
+	static void put_u24_le(std::uint8_t dst[3], std::uint32_t v);
 
 	void enqueue(Pending &&p);
 
@@ -136,9 +161,13 @@ private:
 	// State
 	SwapConfig cfg_{};
 	std::unordered_map<Buffer *, JournalCtx> journals_;
+	std::unordered_map<Buffer *, std::unique_ptr<BufferRecorder> > recorders_;
 	std::mutex mtx_;
 	std::condition_variable cv_;
 	std::vector<Pending> queue_;
+	std::uint64_t next_seq_{0};
+	std::uint64_t last_processed_{0};
+	std::uint64_t inflight_{0};
 	std::atomic<bool> running_{false};
 	std::thread worker_;
 };

SwapRecorder.h

@@ -0,0 +1,19 @@
+// SwapRecorder.h - minimal swap journal recording interface for Buffer mutations
+#pragma once
+
+#include <cstddef>
+#include <string_view>
+
+namespace kte {
+// SwapRecorder is a tiny, non-blocking callback interface.
+// Implementations must return quickly; Buffer calls these hooks after a
+// mutation succeeds.
+class SwapRecorder {
+public:
+	virtual ~SwapRecorder() = default;
+
+	virtual void OnInsert(int row, int col, std::string_view bytes) = 0;
+
+	virtual void OnDelete(int row, int col, std::size_t len) = 0;
+};
+} // namespace kte

TerminalInputHandler.cc

@@ -3,6 +3,7 @@
 
 #include "TerminalInputHandler.h"
 #include "KKeymap.h"
+#include "Command.h"
 #include "Editor.h"
 
 namespace {
@@ -23,6 +24,7 @@ map_key_to_command(const int ch,
                    bool &k_prefix,
                    bool &esc_meta,
                    bool &k_ctrl_pending,
+                   bool &mouse_selecting,
                    Editor *ed,
                    MappedInput &out)
 {
@@ -54,13 +56,34 @@ map_key_to_command(const int ch,
 			}
 #endif
 			// React to left button click/press
-			if (ev.bstate & (BUTTON1_CLICKED | BUTTON1_PRESSED | BUTTON1_RELEASED)) {
+			if (ed && (ev.bstate & (BUTTON1_CLICKED | BUTTON1_PRESSED | BUTTON1_RELEASED |
+			                        REPORT_MOUSE_POSITION))) {
 				char buf[64];
 				// Use screen coordinates; command handler will translate via offsets
 				std::snprintf(buf, sizeof(buf), "@%d:%d", ev.y, ev.x);
-				out = {true, CommandId::MoveCursorTo, std::string(buf), 0};
+				const bool pressed  = (ev.bstate & (BUTTON1_PRESSED | BUTTON1_CLICKED)) != 0;
+				const bool released = (ev.bstate & BUTTON1_RELEASED) != 0;
+				const bool moved    = (ev.bstate & REPORT_MOUSE_POSITION) != 0;
+				if (pressed) {
+					mouse_selecting = true;
+					Execute(*ed, CommandId::MoveCursorTo, std::string(buf));
+					if (Buffer *b = ed->CurrentBuffer()) {
+						b->SetMark(b->Curx(), b->Cury());
+					}
+					out.hasCommand = false;
 					return true;
 				}
+				if (mouse_selecting && moved) {
+					Execute(*ed, CommandId::MoveCursorTo, std::string(buf));
+					out.hasCommand = false;
+					return true;
+				}
+				if (released) {
+					mouse_selecting = false;
+					out.hasCommand  = false;
+					return true;
+				}
+			}
 		}
 		// No actionable mouse event
 		out.hasCommand = false;
@@ -292,6 +315,7 @@ TerminalInputHandler::decode_(MappedInput &out)
 		ch,
 		k_prefix_, esc_meta_,
 		k_ctrl_pending_,
+		mouse_selecting_,
 		ed_,
 		out);
 	if (!consumed)

TerminalInputHandler.h

@@ -30,5 +30,8 @@ private:
 	// Simple meta (ESC) state for ESC sequences like ESC b/f
 	bool esc_meta_ = false;
 
+	// Mouse drag selection state
+	bool mouse_selecting_ = false;
+
 	Editor *ed_ = nullptr; // attached editor for uarg handling
 };

TerminalRenderer.cc

@@ -107,11 +107,80 @@ TerminalRenderer::Draw(Editor &ed)
 			const std::size_t cur_mx   = has_current ? ed.SearchMatchX() : 0;
 			const std::size_t cur_my   = has_current ? ed.SearchMatchY() : 0;
 			const std::size_t cur_mend = has_current ? (ed.SearchMatchX() + ed.SearchMatchLen()) : 0;
-			bool hl_on                 = false;
-			bool cur_on                = false;
+
+			// Mark selection (mark -> cursor), in source coordinates
+			bool sel_active    = false;
+			std::size_t sel_sy = 0, sel_sx = 0, sel_ey = 0, sel_ex = 0;
+			if (buf->MarkSet()) {
+				sel_sy = buf->MarkCury();
+				sel_sx = buf->MarkCurx();
+				sel_ey = buf->Cury();
+				sel_ex = buf->Curx();
+				if (sel_sy > sel_ey || (sel_sy == sel_ey && sel_sx > sel_ex)) {
+					std::swap(sel_sy, sel_ey);
+					std::swap(sel_sx, sel_ex);
+				}
+				sel_active = !(sel_sy == sel_ey && sel_sx == sel_ex);
+			}
+			// Visual-line selection: full-line selection range
+			const bool vsel_active    = buf->VisualLineActive();
+			const std::size_t vsel_sy = vsel_active ? buf->VisualLineStartY() : 0;
+			const std::size_t vsel_ey = vsel_active ? buf->VisualLineEndY() : 0;
+			auto is_src_in_mark_sel   = [&](std::size_t y, std::size_t sx) -> bool {
+				if (!sel_active)
+					return false;
+				if (y < sel_sy || y > sel_ey)
+					return false;
+				if (sel_sy == sel_ey)
+					return sx >= sel_sx && sx < sel_ex;
+				if (y == sel_sy)
+					return sx >= sel_sx;
+				if (y == sel_ey)
+					return sx < sel_ex;
+				return true;
+			};
 			int written = 0;
 			if (li < lines.size()) {
 				std::string line                = static_cast<std::string>(lines[li]);
+				const bool vsel_on_line         = vsel_active && li >= vsel_sy && li <= vsel_ey;
+				const std::size_t vsel_spot_src = vsel_on_line
+					                                  ? std::min(buf->Curx(), line.size())
+					                                  : 0;
+				const bool vsel_spot_is_eol = vsel_on_line && vsel_spot_src == line.size();
+				std::size_t vsel_line_rx    = 0;
+				if (vsel_spot_is_eol) {
+					// Compute the rendered (column) width of the line so we can highlight a
+					// single cell at EOL when the spot falls beyond the last character.
+					std::size_t rc = 0;
+					std::size_t si = 0;
+					while (si < line.size()) {
+						wchar_t wch = L' ';
+						int wch_len = 1;
+						std::mbstate_t state = std::mbstate_t();
+						size_t res = std::mbrtowc(&wch, &line[si], line.size() - si, &state);
+						if (res == (size_t) -1 || res == (size_t) -2) {
+							wch     = static_cast<unsigned char>(line[si]);
+							wch_len = 1;
+						} else if (res == 0) {
+							wch     = L'\0';
+							wch_len = 1;
+						} else {
+							wch_len = static_cast<int>(res);
+						}
+						if (wch == L'\t') {
+							constexpr std::size_t tab_width = 8;
+							const std::size_t next_tab      = tab_width - (rc % tab_width);
+							rc                              += next_tab;
+						} else {
+							int w = wcwidth(wch);
+							if (w < 0)
+								w = 1;
+							rc += static_cast<std::size_t>(w);
+						}
+						si += static_cast<std::size_t>(wch_len);
+					}
+					vsel_line_rx = rc;
+				}
 				src_i      = 0;
 				render_col = 0;
 				// Syntax highlighting: fetch per-line spans (sanitized copy)
@@ -156,27 +225,21 @@ TerminalRenderer::Draw(Editor &ed)
 					}
 					return kte::TokenKind::Default;
 				};
-				auto apply_token_attr = [&](kte::TokenKind k) {
-					// Map to simple attributes; search highlight uses A_STANDOUT which takes precedence below
-					attrset(A_NORMAL);
+				auto token_attr = [&](kte::TokenKind k) -> attr_t {
 					switch (k) {
 					case kte::TokenKind::Keyword:
 					case kte::TokenKind::Type:
 					case kte::TokenKind::Constant:
 					case kte::TokenKind::Function:
-						attron(A_BOLD);
-						break;
+						return A_BOLD;
 					case kte::TokenKind::Comment:
-						attron(A_DIM);
-						break;
+						return A_DIM;
 					case kte::TokenKind::String:
 					case kte::TokenKind::Char:
 					case kte::TokenKind::Number:
-						// standout a bit using A_UNDERLINE if available
-						attron(A_UNDERLINE);
-						break;
+						return A_UNDERLINE;
 					default:
-						break;
+						return 0;
 					}
 				};
 				while (written < cols) {
@@ -218,36 +281,27 @@ TerminalRenderer::Draw(Editor &ed)
 							}
 							// Now render visible spaces
 							while (next_tab > 0 && written < cols) {
+								bool in_mark = is_src_in_mark_sel(li, src_i);
+								bool in_vsel =
+									vsel_on_line && !vsel_spot_is_eol && src_i ==
+									vsel_spot_src;
+								bool in_sel = in_mark || in_vsel;
 								bool in_hl  = search_mode && is_src_in_hl(src_i);
 								bool in_cur =
 									has_current && li == cur_my && src_i >= cur_mx
-									&& src_i < cur_mend;
-								// Toggle highlight attributes
-								int attr = 0;
+									&&
+									src_i < cur_mend;
+								attr_t a = A_NORMAL;
+								a        |= token_attr(token_at(src_i));
+								if (in_sel) {
+									a |= A_REVERSE;
+								} else {
 									if (in_hl)
-									attr |= A_STANDOUT;
+										a |= A_STANDOUT;
 									if (in_cur)
-									attr |= A_BOLD;
-								if ((attr & A_STANDOUT) && !hl_on) {
-									attron(A_STANDOUT);
-									hl_on = true;
-								}
-								if (!(attr & A_STANDOUT) && hl_on) {
-									attroff(A_STANDOUT);
-									hl_on = false;
-								}
-								if ((attr & A_BOLD) && !cur_on) {
-									attron(A_BOLD);
-									cur_on = true;
-								}
-								if (!(attr & A_BOLD) && cur_on) {
-									attroff(A_BOLD);
-									cur_on = false;
-								}
-								// Apply syntax attribute only if not in search highlight
-								if (!in_hl) {
-									apply_token_attr(token_at(src_i));
+										a |= A_BOLD;
 								}
+								attrset(a);
 								addch(' ');
 								++written;
 								++render_col;
@@ -281,34 +335,36 @@ TerminalRenderer::Draw(Editor &ed)
 						break;
 					}
 
+					bool in_mark = from_src && is_src_in_mark_sel(li, src_i);
+					bool in_vsel = false;
+					if (vsel_on_line) {
+						if (from_src) {
+							in_vsel = !vsel_spot_is_eol && src_i == vsel_spot_src;
+						} else {
+							in_vsel = vsel_spot_is_eol && render_col == vsel_line_rx;
+						}
+					}
+					bool in_sel = in_mark || in_vsel;
 					bool in_hl  = search_mode && from_src && is_src_in_hl(src_i);
-					bool in_cur =
-						has_current && li == cur_my && from_src && src_i >= cur_mx && src_i <
-						cur_mend;
-					if (in_hl && !hl_on) {
-						attron(A_STANDOUT);
-						hl_on = true;
-					}
-					if (!in_hl && hl_on) {
-						attroff(A_STANDOUT);
-						hl_on = false;
-					}
-					if (in_cur && !cur_on) {
-						attron(A_BOLD);
-						cur_on = true;
-					}
-					if (!in_cur && cur_on) {
-						attroff(A_BOLD);
-						cur_on = false;
-					}
-					if (!in_hl && from_src) {
-						apply_token_attr(token_at(src_i));
+					bool in_cur = has_current && li == cur_my && from_src && src_i >= cur_mx &&
+					              src_i < cur_mend;
+					attr_t a = A_NORMAL;
+					if (from_src)
+						a |= token_attr(token_at(src_i));
+					if (in_sel) {
+						a |= A_REVERSE;
+					} else {
+						if (in_hl)
+							a |= A_STANDOUT;
+						if (in_cur)
+							a |= A_BOLD;
 					}
+					attrset(a);
 
 					if (from_src) {
 						cchar_t cch;
 						wchar_t warr[2] = {wch, L'\0'};
-						setcchar(&cch, warr, A_NORMAL, 0, nullptr);
+						setcchar(&cch, warr, 0, 0, nullptr);
 						add_wch(&cch);
 					} else {
 						addch(' ');
@@ -322,14 +378,6 @@ TerminalRenderer::Draw(Editor &ed)
 						break;
 				}
 			}
-			if (hl_on) {
-				attroff(A_STANDOUT);
-				hl_on = false;
-			}
-			if (cur_on) {
-				attroff(A_BOLD);
-				cur_on = false;
-			}
 			attrset(A_NORMAL);
 			clrtoeol();
 		}

UndoNode.h

@@ -15,7 +15,9 @@ struct UndoNode {
 	UndoType type{};
 	int row{};
 	int col{};
+	std::uint64_t group_id = 0; // 0 means ungrouped; non-zero means undo/redo as an atomic group
 	std::string text;
+	UndoNode *parent = nullptr; // previous state; null means pre-first-edit
 	UndoNode *child  = nullptr; // next in current timeline
 	UndoNode *next   = nullptr; // redo branch
 };

UndoNodePool.h

@@ -20,6 +20,7 @@ public:
 		available_.pop();
 		// Node comes zeroed; ensure links are reset
 		node->text.clear();
+		node->parent = nullptr;
 		node->child  = nullptr;
 		node->next   = nullptr;
 		node->row    = node->col = 0;
@@ -34,6 +35,7 @@ public:
 			return;
 		// Clear heavy fields to free memory held by strings
 		node->text.clear();
+		node->parent = nullptr;
 		node->child  = nullptr;
 		node->next   = nullptr;
 		node->row    = node->col = 0;

UndoSystem.cc

@@ -8,69 +8,262 @@ UndoSystem::UndoSystem(Buffer &owner, UndoTree &tree)
 	: buf_(&owner), tree_(tree) {}
 
 
+std::uint64_t
+UndoSystem::BeginGroup()
+{
+	// Ensure any pending typed run is sealed so the group is a distinct undo step.
+	commit();
+	if (active_group_id_ == 0)
+		active_group_id_ = next_group_id_++;
+	return active_group_id_;
+}
+
+
+void
+UndoSystem::EndGroup()
+{
+	commit();
+	active_group_id_ = 0;
+}
+
+
 void
 UndoSystem::Begin(UndoType type)
 {
-	// STUB: Undo system incomplete - disabled until it can be properly implemented
-	(void) type;
+	if (!buf_)
+		return;
+	const int row = static_cast<int>(buf_->Cury());
+	const int col = static_cast<int>(buf_->Curx());
+
+	// Some operations should always be standalone undo steps.
+	const bool always_standalone = (type == UndoType::Newline || type == UndoType::DeleteRow);
+	if (always_standalone) {
+		commit();
+	}
+
+	if (tree_.pending) {
+		if (tree_.pending->type == type) {
+			// Typed-run coalescing rules.
+			switch (type) {
+			case UndoType::Insert:
+			case UndoType::Paste: {
+				// Cursor must be at the end of the pending insert.
+				if (tree_.pending->row == row
+				    && col == tree_.pending->col + static_cast<int>(tree_.pending->text.size())) {
+					pending_mode_ = PendingAppendMode::Append;
+					return;
+				}
+				break;
+			}
+			case UndoType::Delete: {
+				if (tree_.pending->row == row) {
+					// Two common delete shapes:
+					// 1) backspace-run: cursor moves left each time (so new col is pending.col - 1)
+					// 2) delete-run: cursor stays, always deleting at the same col
+					if (col == tree_.pending->col) {
+						pending_mode_ = PendingAppendMode::Append;
+						return;
+					}
+					if (col + 1 == tree_.pending->col) {
+						// Extend a backspace run to the left; update the start column now.
+						tree_.pending->col = col;
+						pending_mode_      = PendingAppendMode::Prepend;
+						return;
+					}
+				}
+				break;
+			}
+			case UndoType::Newline:
+			case UndoType::DeleteRow:
+				break;
+			}
+		}
+		// Can't coalesce: seal the previous pending step.
+		commit();
+	}
+
+	// Start a new pending node.
+	tree_.pending           = new UndoNode{};
+	tree_.pending->type     = type;
+	tree_.pending->row      = row;
+	tree_.pending->col      = col;
+	tree_.pending->group_id = active_group_id_;
+	tree_.pending->text.clear();
+	tree_.pending->parent = nullptr;
+	tree_.pending->child  = nullptr;
+	tree_.pending->next   = nullptr;
+	pending_mode_         = PendingAppendMode::Append;
 }
 
 
 void
 UndoSystem::Append(char ch)
 {
-	// STUB: Undo system incomplete - disabled until it can be properly implemented
-	(void) ch;
+	if (!tree_.pending)
+		return;
+	if (pending_mode_ == PendingAppendMode::Prepend) {
+		tree_.pending->text.insert(tree_.pending->text.begin(), ch);
+	} else {
+		tree_.pending->text.push_back(ch);
+	}
 }
 
 
 void
 UndoSystem::Append(std::string_view text)
 {
-	// STUB: Undo system incomplete - disabled until it can be properly implemented
-	(void) text;
+	if (!tree_.pending)
+		return;
+	if (text.empty())
+		return;
+	if (pending_mode_ == PendingAppendMode::Prepend) {
+		tree_.pending->text.insert(0, text.data(), text.size());
+	} else {
+		tree_.pending->text.append(text.data(), text.size());
+	}
 }
 
 
 void
 UndoSystem::commit()
 {
-	// STUB: Undo system incomplete - disabled until it can be properly implemented
+	if (!tree_.pending)
+		return;
+
+	// Drop empty text batches for text-based operations.
+	if ((tree_.pending->type == UndoType::Insert || tree_.pending->type == UndoType::Delete
+	     || tree_.pending->type == UndoType::Paste)
+	    && tree_.pending->text.empty()) {
+		delete tree_.pending;
+		tree_.pending = nullptr;
+		pending_mode_ = PendingAppendMode::Append;
+		return;
+	}
+
+	if (!tree_.root) {
+		tree_.root            = tree_.pending;
+		tree_.pending->parent = nullptr;
+		tree_.current         = tree_.pending;
+	} else if (!tree_.current) {
+		// We are at the "pre-first-edit" state (undo past the first node).
+		// In branching history, preserve the existing root chain as an alternate branch.
+		tree_.pending->parent = nullptr;
+		tree_.pending->next   = tree_.root;
+		tree_.root            = tree_.pending;
+		tree_.current         = tree_.pending;
+	} else {
+		// Branching semantics: attach as a new redo branch under current.
+		// Make the new edit the active child by inserting it at the head.
+		tree_.pending->parent = tree_.current;
+		if (!tree_.current->child) {
+			tree_.current->child = tree_.pending;
+		} else {
+			tree_.pending->next  = tree_.current->child;
+			tree_.current->child = tree_.pending;
+		}
+		tree_.current = tree_.pending;
+	}
+
+	tree_.pending = nullptr;
+	pending_mode_ = PendingAppendMode::Append;
+	update_dirty_flag();
 }
 
 
 void
 UndoSystem::undo()
 {
-	// STUB: Undo system incomplete - disabled until it can be properly implemented
+	// Seal any in-progress typed run before undo.
+	commit();
+	if (!tree_.current)
+		return;
+	debug_log("undo");
+	const std::uint64_t gid = tree_.current->group_id;
+	do {
+		UndoNode *node = tree_.current;
+		apply(node, -1);
+		tree_.current = node->parent;
+	} while (gid != 0 && tree_.current && tree_.current->group_id == gid);
+	update_dirty_flag();
 }
 
 
 void
-UndoSystem::redo()
+UndoSystem::redo(int branch_index)
 {
-	// STUB: Undo system incomplete - disabled until it can be properly implemented
+	commit();
+	UndoNode **head = nullptr;
+	if (!tree_.current) {
+		head = &tree_.root;
+	} else {
+		head = &tree_.current->child;
+	}
+	if (!head || !*head)
+		return;
+	if (branch_index < 0)
+		branch_index = 0;
+
+	// Select the Nth sibling from the branch list and make it the active head.
+	UndoNode *prev = nullptr;
+	UndoNode *sel  = *head;
+	for (int i = 0; i < branch_index && sel; ++i) {
+		prev = sel;
+		sel  = sel->next;
+	}
+	if (!sel)
+		return;
+	if (prev) {
+		prev->next = sel->next;
+		sel->next  = *head;
+		*head      = sel;
+	}
+
+	debug_log("redo");
+	UndoNode *node          = *head;
+	const std::uint64_t gid = node->group_id;
+	apply(node, +1);
+	tree_.current = node;
+	while (gid != 0 && tree_.current && tree_.current->child
+	       && tree_.current->child->group_id == gid) {
+		UndoNode *child = tree_.current->child;
+		apply(child, +1);
+		tree_.current = child;
+	}
+	update_dirty_flag();
 }
 
 
 void
 UndoSystem::mark_saved()
 {
-	// STUB: Undo system incomplete - disabled until it can be properly implemented
+	commit();
+	tree_.saved = tree_.current;
+	update_dirty_flag();
 }
 
 
 void
 UndoSystem::discard_pending()
 {
-	// STUB: Undo system incomplete - disabled until it can be properly implemented
+	if (tree_.pending) {
+		delete tree_.pending;
+		tree_.pending = nullptr;
+	}
+	pending_mode_ = PendingAppendMode::Append;
 }
 
 
 void
 UndoSystem::clear()
 {
-	// STUB: Undo system incomplete - disabled until it can be properly implemented
+	discard_pending();
+	free_node(tree_.root);
+	tree_.root       = nullptr;
+	tree_.current    = nullptr;
+	tree_.saved      = nullptr;
+	active_group_id_ = 0;
+	next_group_id_   = 1;
+	update_dirty_flag();
 }
 
 
@@ -79,34 +272,46 @@ UndoSystem::apply(const UndoNode *node, int direction)
 {
 	if (!node)
 		return;
+	// Cursor positioning: keep the point at a sensible location after undo/redo.
+	// Low-level Buffer edit primitives do not move the cursor.
 	switch (node->type) {
 	case UndoType::Insert:
 	case UndoType::Paste:
 		if (direction > 0) {
 			buf_->insert_text(node->row, node->col, node->text);
+			buf_->SetCursor(static_cast<std::size_t>(node->col + node->text.size()),
+			                static_cast<std::size_t>(node->row));
 		} else {
 			buf_->delete_text(node->row, node->col, node->text.size());
+			buf_->SetCursor(static_cast<std::size_t>(node->col), static_cast<std::size_t>(node->row));
 		}
 		break;
 	case UndoType::Delete:
 		if (direction > 0) {
 			buf_->delete_text(node->row, node->col, node->text.size());
+			buf_->SetCursor(static_cast<std::size_t>(node->col), static_cast<std::size_t>(node->row));
 		} else {
 			buf_->insert_text(node->row, node->col, node->text);
+			buf_->SetCursor(static_cast<std::size_t>(node->col + node->text.size()),
+			                static_cast<std::size_t>(node->row));
 		}
 		break;
 	case UndoType::Newline:
 		if (direction > 0) {
 			buf_->split_line(node->row, node->col);
+			buf_->SetCursor(0, static_cast<std::size_t>(node->row + 1));
 		} else {
 			buf_->join_lines(node->row);
+			buf_->SetCursor(static_cast<std::size_t>(node->col), static_cast<std::size_t>(node->row));
 		}
 		break;
 	case UndoType::DeleteRow:
 		if (direction > 0) {
 			buf_->delete_row(node->row);
+			buf_->SetCursor(0, static_cast<std::size_t>(node->row));
 		} else {
 			buf_->insert_row(node->row, node->text);
+			buf_->SetCursor(0, static_cast<std::size_t>(node->row));
 		}
 		break;
 	}

UndoSystem.h

@@ -12,6 +12,12 @@ class UndoSystem {
 public:
 	explicit UndoSystem(Buffer &owner, UndoTree &tree);
 
+	// Begin an atomic group: subsequent committed nodes with the same group_id will be
+	// undone/redone as a single step. Returns the active group id.
+	std::uint64_t BeginGroup();
+
+	void EndGroup();
+
 	void Begin(UndoType type);
 
 	void Append(char ch);
@@ -22,7 +28,10 @@ public:
 
 	void undo();
 
-	void redo();
+	// Redo the current node's active child branch.
+	// If `branch_index` > 0, selects that redo sibling (0-based) and makes it active.
+	// When current is null (pre-first-edit), branches are selected among `tree_.root` siblings.
+	void redo(int branch_index = 0);
 
 	void mark_saved();
 
@@ -32,7 +41,20 @@ public:
 
 	void UpdateBufferReference(Buffer &new_buf);
 
+#if defined(KTE_TESTS)
+	// Test-only introspection hook.
+	const UndoTree &TreeForTests() const
+	{
+		return tree_;
+	}
+#endif
+
 private:
+	enum class PendingAppendMode : std::uint8_t {
+		Append,
+		Prepend,
+	};
+
 	void apply(const UndoNode *node, int direction); // +1 redo, -1 undo
 	void free_node(UndoNode *node);
 
@@ -48,6 +70,11 @@ private:
 
 	void update_dirty_flag();
 
+	PendingAppendMode pending_mode_ = PendingAppendMode::Append;
+
+	std::uint64_t active_group_id_ = 0;
+	std::uint64_t next_group_id_   = 1;
+
 	Buffer *buf_;
 	UndoTree &tree_;
 };

docs/plans/swap-files.md

@@ -12,11 +12,14 @@ Goals
 
 Model overview
 --------------
-Per open buffer, maintain a sidecar swap journal next to the file:
+Per open buffer, maintain a swap journal in a per-user state directory:
 
-- Path: `.<basename>.kte.swp` in the same directory as the file (for
-  unnamed/unsaved buffers, use a per‑session temp dir like
-  `$TMPDIR/kte/` with a random UUID).
+- Path: `$XDG_STATE_HOME/kte/swap/<encoded-path>.swp` (or
+  `~/.local/state/kte/swap/...`)
+  where `<encoded-path>` is the file path with separators replaced (e.g.
+  `/home/kyle/tmp/test.txt` → `home!kyle!tmp!test.txt.swp`).
+  Unnamed/unsaved
+  buffers use a unique `unnamed-<pid>-<counter>.swp` name.
 - Format: append‑only journal of editing operations with periodic
   checkpoints.
 - Crash safety: only append, fsync as per policy; checkpoint via
@@ -84,7 +87,7 @@ Recovery flow
 
 On opening a file:
 
-1. Detect swap sidecar `.<basename>.kte.swp`.
+1. Detect swap journal `$XDG_STATE_HOME/kte/swap/<encoded-path>.swp`.
 2. Validate header, iterate records verifying CRCs.
 3. Compare recorded original file identity against actual file; if
    mismatch, warn user but allow recovery (content wins).
@@ -98,7 +101,7 @@ Stability & corruption mitigation
 ---------------------------------
 
 - Append‑only with per‑record CRC32 guards against torn writes.
-- Atomic checkpoint rotation: write `.<basename>.kte.swp.tmp`, fsync,
+- Atomic checkpoint rotation: write `<encoded-path>.swp.tmp`, fsync,
   then rename over old `.swp`.
 - Size caps: rotate or compact when `.swp` exceeds a threshold (e.g.,
   64–128 MB). Compaction creates a fresh file with a single checkpoint.
@@ -117,8 +120,8 @@ Security considerations
 Interoperability & UX
 ---------------------
 
-- Use a distinctive extension `.kte.swp` to avoid conflicts with other
-  editors.
+- Use a distinctive directory (`$XDG_STATE_HOME/kte/swap`) to avoid
+  conflicts with other editors’ `.swp` conventions.
 - Status bar indicator when swap is active; commands to purge/compact.
 - On save: do not delete swap immediately; keep until the buffer is
   clean and idle for a short grace period (allows undo of accidental

tests/TestHarness.h

@@ -0,0 +1,138 @@
+// TestHarness.h - small helper layer for driving kte headlessly in tests
+#pragma once
+
+#include <cstddef>
+#include <string>
+#include <string_view>
+
+#include "Buffer.h"
+#include "Command.h"
+#include "Editor.h"
+
+namespace ktet {
+inline void
+InstallDefaultCommandsOnce()
+{
+	static bool installed = false;
+	if (!installed) {
+		InstallDefaultCommands();
+		installed = true;
+	}
+}
+
+
+class TestHarness {
+public:
+	TestHarness()
+	{
+		InstallDefaultCommandsOnce();
+		editor_.SetDimensions(24, 80);
+
+		Buffer b;
+		b.SetVirtualName("+TEST+");
+		editor_.AddBuffer(std::move(b));
+	}
+
+
+	Editor &
+	EditorRef()
+	{
+		return editor_;
+	}
+
+
+	Buffer &
+	Buf()
+	{
+		return *editor_.CurrentBuffer();
+	}
+
+
+	[[nodiscard]] const Buffer &
+	Buf() const
+	{
+		return *editor_.CurrentBuffer();
+	}
+
+
+	bool
+	Exec(CommandId id, const std::string &arg = std::string(), int ucount = 0)
+	{
+		if (ucount > 0) {
+			editor_.SetUniversalArg(1, ucount);
+		} else {
+			editor_.UArgClear();
+		}
+		return Execute(editor_, id, arg);
+	}
+
+
+	bool
+	InsertText(std::string_view text)
+	{
+		if (text.find('\n') != std::string_view::npos || text.find('\r') != std::string_view::npos)
+			return false;
+		return Exec(CommandId::InsertText, std::string(text));
+	}
+
+
+	void
+	TypeText(std::string_view text)
+	{
+		for (char ch: text) {
+			if (ch == '\n') {
+				Exec(CommandId::Newline);
+			} else if (ch == '\r') {
+				// ignore
+			} else {
+				Exec(CommandId::InsertText, std::string(1, ch));
+			}
+		}
+	}
+
+
+	[[nodiscard]] std::string
+	Text() const
+	{
+		const auto &rows = Buf().Rows();
+		std::string out;
+		for (std::size_t i = 0; i < rows.size(); ++i) {
+			out += static_cast<std::string>(rows[i]);
+			if (i + 1 < rows.size())
+				out.push_back('\n');
+		}
+		return out;
+	}
+
+
+	[[nodiscard]] std::string
+	Line(std::size_t y) const
+	{
+		return Buf().GetLineString(y);
+	}
+
+
+	bool
+	SaveAs(const std::string &path, std::string &err)
+	{
+		return Buf().SaveAs(path, err);
+	}
+
+
+	bool
+	Undo(int ucount = 0)
+	{
+		return Exec(CommandId::Undo, std::string(), ucount);
+	}
+
+
+	bool
+	Redo(int ucount = 0)
+	{
+		return Exec(CommandId::Redo, std::string(), ucount);
+	}
+
+private:
+	Editor editor_;
+};
+} // namespace ktet

tests/test_buffer_rows.cc

@@ -0,0 +1,142 @@
+#include "Test.h"
+#include "Buffer.h"
+#include <algorithm>
+#include <limits>
+#include <string>
+#include <vector>
+
+
+static std::vector<std::string>
+split_lines_preserve_trailing_empty(const std::string &s)
+{
+	std::vector<std::string> out;
+	std::size_t start = 0;
+	for (std::size_t i = 0; i <= s.size(); i++) {
+		if (i == s.size() || s[i] == '\n') {
+			out.push_back(s.substr(start, i - start));
+			start = i + 1;
+		}
+	}
+	if (out.empty())
+		out.push_back(std::string());
+	return out;
+}
+
+
+static std::vector<std::size_t>
+line_starts_for(const std::string &s)
+{
+	std::vector<std::size_t> starts;
+	starts.push_back(0);
+	for (std::size_t i = 0; i < s.size(); i++) {
+		if (s[i] == '\n')
+			starts.push_back(i + 1);
+	}
+	return starts;
+}
+
+
+static std::size_t
+ref_linecol_to_offset(const std::string &s, std::size_t row, std::size_t col)
+{
+	auto starts = line_starts_for(s);
+	if (starts.empty())
+		return 0;
+	if (row >= starts.size())
+		return s.size();
+	std::size_t start = starts[row];
+	std::size_t end   = (row + 1 < starts.size()) ? starts[row + 1] : s.size();
+	if (end > start && s[end - 1] == '\n')
+		end -= 1; // clamp before trailing newline
+	return start + std::min(col, end - start);
+}
+
+
+static void
+check_buffer_matches_model(const Buffer &b, const std::string &model)
+{
+	auto expected_lines = split_lines_preserve_trailing_empty(model);
+	const auto &rows    = b.Rows();
+	ASSERT_EQ(rows.size(), expected_lines.size());
+	ASSERT_EQ(b.Nrows(), rows.size());
+
+	auto starts = line_starts_for(model);
+	ASSERT_EQ(starts.size(), expected_lines.size());
+
+	std::string via_views;
+	for (std::size_t i = 0; i < rows.size(); i++) {
+		ASSERT_EQ(std::string(rows[i]), expected_lines[i]);
+		ASSERT_EQ(b.GetLineString(i), expected_lines[i]);
+
+		std::size_t exp_start = starts[i];
+		std::size_t exp_end   = (i + 1 < starts.size()) ? starts[i + 1] : model.size();
+		auto r                = b.GetLineRange(i);
+		ASSERT_EQ(r.first, exp_start);
+		ASSERT_EQ(r.second, exp_end);
+
+		auto v = b.GetLineView(i);
+		ASSERT_EQ(std::string(v), model.substr(exp_start, exp_end - exp_start));
+		via_views.append(v.data(), v.size());
+	}
+	ASSERT_EQ(via_views, model);
+}
+
+
+TEST (Buffer_RowsCache_MultiLineEdits_StayConsistent)
+{
+	Buffer b;
+	std::string model;
+
+	check_buffer_matches_model(b, model);
+
+	// Insert text and newlines in a few different ways.
+	b.insert_text(0, 0, std::string("abc"));
+	model.insert(0, "abc");
+	check_buffer_matches_model(b, model);
+
+	b.split_line(0, 1); // a\nbc
+	model.insert(ref_linecol_to_offset(model, 0, 1), "\n");
+	check_buffer_matches_model(b, model);
+
+	b.insert_text(1, 2, std::string("X")); // a\nbcX
+	model.insert(ref_linecol_to_offset(model, 1, 2), "X");
+	check_buffer_matches_model(b, model);
+
+	b.join_lines(0); // abcX
+	{
+		std::size_t off = ref_linecol_to_offset(model, 0, std::numeric_limits<std::size_t>::max());
+		if (off < model.size() && model[off] == '\n')
+			model.erase(off, 1);
+	}
+	check_buffer_matches_model(b, model);
+
+	// Insert a multi-line segment in one shot.
+	b.insert_text(0, 2, std::string("\n123\nxyz"));
+	model.insert(ref_linecol_to_offset(model, 0, 2), "\n123\nxyz");
+	check_buffer_matches_model(b, model);
+
+	// Delete spanning across a newline.
+	b.delete_text(0, 1, 5);
+	{
+		std::size_t start  = ref_linecol_to_offset(model, 0, 1);
+		std::size_t actual = std::min<std::size_t>(5, model.size() - start);
+		model.erase(start, actual);
+	}
+	check_buffer_matches_model(b, model);
+
+	// Insert/delete whole rows.
+	b.insert_row(1, std::string_view("ROW"));
+	model.insert(ref_linecol_to_offset(model, 1, 0), "ROW\n");
+	check_buffer_matches_model(b, model);
+
+	b.delete_row(1);
+	{
+		auto starts = line_starts_for(model);
+		if (1 < (int) starts.size()) {
+			std::size_t start = starts[1];
+			std::size_t end   = (2 < starts.size()) ? starts[2] : model.size();
+			model.erase(start, end - start);
+		}
+	}
+	check_buffer_matches_model(b, model);
+}

tests/test_command_semantics.cc

@@ -0,0 +1,91 @@
+#include "Test.h"
+
+#include "TestHarness.h"
+
+using ktet::TestHarness;
+
+
+TEST (CommandSemantics_KillToEOL_KillChain_And_Yank)
+{
+	TestHarness h;
+	Editor &ed = h.EditorRef();
+	Buffer &b  = h.Buf();
+
+	b.insert_text(0, 0, std::string("abc\ndef"));
+	b.SetCursor(1, 0); // a|bc
+
+	ed.KillRingClear();
+	ed.SetKillChain(false);
+
+	ASSERT_TRUE(h.Exec(CommandId::KillToEOL));
+	ASSERT_EQ(h.Text(), std::string("a\ndef"));
+	ASSERT_EQ(ed.KillRingHead(), std::string("bc"));
+
+	// At EOL, KillToEOL kills the newline (join).
+	ASSERT_TRUE(h.Exec(CommandId::KillToEOL));
+	ASSERT_EQ(h.Text(), std::string("adef"));
+	ASSERT_EQ(ed.KillRingHead(), std::string("bc\n"));
+
+	// Yank pastes the kill ring head and breaks the kill chain.
+	ASSERT_TRUE(h.Exec(CommandId::Yank));
+	ASSERT_EQ(h.Text(), std::string("abc\ndef"));
+	ASSERT_EQ(ed.KillRingHead(), std::string("bc\n"));
+	ASSERT_EQ(ed.KillChain(), false);
+}
+
+
+TEST (CommandSemantics_ToggleMark_JumpToMark)
+{
+	TestHarness h;
+	Buffer &b = h.Buf();
+
+	b.insert_text(0, 0, std::string("hello"));
+	b.SetCursor(2, 0);
+	ASSERT_EQ(b.MarkSet(), false);
+
+	ASSERT_TRUE(h.Exec(CommandId::ToggleMark));
+	ASSERT_EQ(b.MarkSet(), true);
+	ASSERT_EQ(b.MarkCurx(), (std::size_t) 2);
+	ASSERT_EQ(b.MarkCury(), (std::size_t) 0);
+
+	b.SetCursor(4, 0);
+	ASSERT_TRUE(h.Exec(CommandId::JumpToMark));
+	ASSERT_EQ(b.Curx(), (std::size_t) 2);
+	ASSERT_EQ(b.Cury(), (std::size_t) 0);
+	// Jump-to-mark swaps: mark becomes previous cursor.
+	ASSERT_EQ(b.MarkSet(), true);
+	ASSERT_EQ(b.MarkCurx(), (std::size_t) 4);
+	ASSERT_EQ(b.MarkCury(), (std::size_t) 0);
+}
+
+
+TEST (CommandSemantics_CopyRegion_And_KillRegion)
+{
+	TestHarness h;
+	Editor &ed = h.EditorRef();
+	Buffer &b  = h.Buf();
+
+	b.insert_text(0, 0, std::string("hello world"));
+	b.SetCursor(0, 0);
+
+	ed.KillRingClear();
+	ed.SetKillChain(false);
+
+	// Copy "hello" (region [0,5)).
+	ASSERT_TRUE(h.Exec(CommandId::ToggleMark));
+	b.SetCursor(5, 0);
+	ASSERT_TRUE(h.Exec(CommandId::CopyRegion));
+	ASSERT_EQ(ed.KillRingHead(), std::string("hello"));
+	ASSERT_EQ(b.MarkSet(), false);
+	ASSERT_EQ(h.Text(), std::string("hello world"));
+
+	// Kill "world" (region [6,11)).
+	ed.SetKillChain(false);
+	b.SetCursor(6, 0);
+	ASSERT_TRUE(h.Exec(CommandId::ToggleMark));
+	b.SetCursor(11, 0);
+	ASSERT_TRUE(h.Exec(CommandId::KillRegion));
+	ASSERT_EQ(ed.KillRingHead(), std::string("world"));
+	ASSERT_EQ(b.MarkSet(), false);
+	ASSERT_EQ(h.Text(), std::string("hello "));
+}

tests/test_daily_driver_harness.cc

@@ -0,0 +1,12 @@
+#include "Test.h"
+
+#include "tests/TestHarness.h"
+
+
+TEST (DailyDriverHarness_Smoke_CanCreateBufferAndInsertText)
+{
+	ktet::TestHarness h;
+
+	ASSERT_TRUE(h.InsertText("hello"));
+	ASSERT_EQ(h.Line(0), std::string("hello"));
+}

tests/test_daily_workflows.cc

@@ -0,0 +1,170 @@
+#include "Test.h"
+
+#include "Command.h"
+#include "Editor.h"
+
+#include "tests/TestHarness.h" // for ktet::InstallDefaultCommandsOnce
+
+#include <cstdio>
+#include <filesystem>
+#include <fstream>
+#include <string>
+
+
+static void
+write_file_bytes(const std::string &path, const std::string &bytes)
+{
+	std::ofstream out(path, std::ios::binary | std::ios::trunc);
+	out.write(bytes.data(), (std::streamsize) bytes.size());
+}
+
+
+static std::string
+read_file_bytes(const std::string &path)
+{
+	std::ifstream in(path, std::ios::binary);
+	return std::string((std::istreambuf_iterator<char>(in)), std::istreambuf_iterator<char>());
+}
+
+
+static std::string
+buffer_bytes_via_views(const Buffer &b)
+{
+	const auto &rows = b.Rows();
+	std::string out;
+	for (std::size_t i = 0; i < rows.size(); i++) {
+		auto v = b.GetLineView(i);
+		out.append(v.data(), v.size());
+	}
+	return out;
+}
+
+
+TEST (DailyWorkflow_OpenEditSave_Transcript)
+{
+	ktet::InstallDefaultCommandsOnce();
+
+	const std::string path = "./.kte_ut_daily_open_edit_save.txt";
+	std::remove(path.c_str());
+	write_file_bytes(path, "one\n");
+	const std::string npath = std::filesystem::canonical(path).string();
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+	// Seed an empty buffer so OpenFile can reuse it.
+	{
+		Buffer scratch;
+		ed.AddBuffer(std::move(scratch));
+	}
+
+	std::string err;
+	ASSERT_TRUE(ed.OpenFile(path, err));
+	ASSERT_TRUE(ed.CurrentBuffer() != nullptr);
+	ASSERT_EQ(ed.CurrentBuffer()->Filename(), npath);
+
+	// Append two new lines via commands (no UI).
+	ASSERT_TRUE(Execute(ed, CommandId::MoveFileEnd));
+	ASSERT_TRUE(Execute(ed, CommandId::InsertText, "two"));
+	ASSERT_TRUE(Execute(ed, CommandId::Newline));
+	ASSERT_TRUE(Execute(ed, CommandId::InsertText, "three"));
+
+	ASSERT_TRUE(Execute(ed, CommandId::Save));
+	ASSERT_TRUE(ed.CurrentBuffer() != nullptr);
+	ASSERT_EQ(read_file_bytes(npath), buffer_bytes_via_views(*ed.CurrentBuffer()));
+
+	std::remove(path.c_str());
+	std::remove(npath.c_str());
+}
+
+
+TEST (DailyWorkflow_MultiBufferSwitchClose_Transcript)
+{
+	ktet::InstallDefaultCommandsOnce();
+
+	const std::string p1 = "./.kte_ut_daily_buf_1.txt";
+	const std::string p2 = "./.kte_ut_daily_buf_2.txt";
+	std::remove(p1.c_str());
+	std::remove(p2.c_str());
+	write_file_bytes(p1, "aaa\n");
+	write_file_bytes(p2, "bbb\n");
+	const std::string np1 = std::filesystem::canonical(p1).string();
+	const std::string np2 = std::filesystem::canonical(p2).string();
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+	{
+		Buffer scratch;
+		ed.AddBuffer(std::move(scratch));
+	}
+
+	std::string err;
+	ASSERT_TRUE(ed.OpenFile(p1, err));
+	ASSERT_TRUE(ed.OpenFile(p2, err));
+	ASSERT_EQ(ed.BufferCount(), (std::size_t) 2);
+	ASSERT_TRUE(ed.CurrentBuffer() != nullptr);
+	ASSERT_EQ(ed.CurrentBuffer()->Filename(), np2);
+
+	// Switch back and forth.
+	ASSERT_TRUE(Execute(ed, CommandId::BufferPrev));
+	ASSERT_EQ(ed.CurrentBuffer()->Filename(), np1);
+	ASSERT_TRUE(Execute(ed, CommandId::BufferNext));
+	ASSERT_EQ(ed.CurrentBuffer()->Filename(), np2);
+
+	// Close current buffer (p2); ensure we land on p1.
+	ASSERT_TRUE(Execute(ed, CommandId::BufferClose));
+	ASSERT_EQ(ed.BufferCount(), (std::size_t) 1);
+	ASSERT_TRUE(ed.CurrentBuffer() != nullptr);
+	ASSERT_EQ(ed.CurrentBuffer()->Filename(), np1);
+
+	std::remove(p1.c_str());
+	std::remove(p2.c_str());
+	std::remove(np1.c_str());
+	std::remove(np2.c_str());
+}
+
+
+TEST (DailyWorkflow_CrashRecovery_SwapReplay_Transcript)
+{
+	ktet::InstallDefaultCommandsOnce();
+
+	const std::string path = "./.kte_ut_daily_swap_recover.txt";
+	std::remove(path.c_str());
+	write_file_bytes(path, "base\nline2\n");
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+	{
+		Buffer scratch;
+		ed.AddBuffer(std::move(scratch));
+	}
+
+	std::string err;
+	ASSERT_TRUE(ed.OpenFile(path, err));
+	Buffer *buf = ed.CurrentBuffer();
+	ASSERT_TRUE(buf != nullptr);
+
+	// Make unsaved edits through command execution.
+	ASSERT_TRUE(Execute(ed, CommandId::MoveFileStart));
+	ASSERT_TRUE(Execute(ed, CommandId::InsertText, "X"));
+	ASSERT_TRUE(Execute(ed, CommandId::MoveDown));
+	ASSERT_TRUE(Execute(ed, CommandId::MoveHome));
+	ASSERT_TRUE(Execute(ed, CommandId::InsertText, "ZZ"));
+	ASSERT_TRUE(Execute(ed, CommandId::MoveFileEnd));
+	ASSERT_TRUE(Execute(ed, CommandId::InsertText, "TAIL"));
+
+	// Ensure journal is durable and capture expected bytes.
+	ed.Swap()->Flush(buf);
+	const std::string swap_path = kte::SwapManager::ComputeSwapPathForTests(*buf);
+	const std::string expected  = buffer_bytes_via_views(*buf);
+
+	// "Crash": reopen from disk (original file content) into a fresh Buffer and replay.
+	Buffer recovered;
+	ASSERT_TRUE(recovered.OpenFromFile(path, err));
+	ASSERT_TRUE(kte::SwapManager::ReplayFile(recovered, swap_path, err));
+	ASSERT_EQ(buffer_bytes_via_views(recovered), expected);
+
+	// Cleanup.
+	ed.Swap()->Detach(buf);
+	std::remove(path.c_str());
+	std::remove(swap_path.c_str());
+}

tests/test_kkeymap.cc

@@ -0,0 +1,84 @@
+#include "Test.h"
+
+#include "KKeymap.h"
+
+#include <ncurses.h>
+
+
+TEST (KKeymap_KPrefix_CanonicalChords)
+{
+	CommandId id{};
+
+	// From docs/ke.md (K-commands)
+	ASSERT_TRUE(KLookupKCommand('s', false, id));
+	ASSERT_EQ(id, CommandId::Save);
+	ASSERT_TRUE(KLookupKCommand('s', true, id)); // C-k C-s
+	ASSERT_EQ(id, CommandId::Save);
+
+	ASSERT_TRUE(KLookupKCommand('d', false, id));
+	ASSERT_EQ(id, CommandId::KillToEOL);
+	ASSERT_TRUE(KLookupKCommand('d', true, id)); // C-k C-d
+	ASSERT_EQ(id, CommandId::KillLine);
+
+	ASSERT_TRUE(KLookupKCommand(' ', false, id)); // C-k SPACE
+	ASSERT_EQ(id, CommandId::ToggleMark);
+
+	ASSERT_TRUE(KLookupKCommand('j', false, id));
+	ASSERT_EQ(id, CommandId::JumpToMark);
+
+	ASSERT_TRUE(KLookupKCommand('f', false, id));
+	ASSERT_EQ(id, CommandId::FlushKillRing);
+
+	ASSERT_TRUE(KLookupKCommand('y', false, id));
+	ASSERT_EQ(id, CommandId::Yank);
+
+	// Unknown should not map
+	ASSERT_EQ(KLookupKCommand('Z', false, id), false);
+}
+
+
+TEST (KKeymap_CtrlChords_CanonicalChords)
+{
+	CommandId id{};
+
+	// From docs/ke.md (other keybindings)
+	ASSERT_TRUE(KLookupCtrlCommand('n', id));
+	ASSERT_EQ(id, CommandId::MoveDown);
+	ASSERT_TRUE(KLookupCtrlCommand('p', id));
+	ASSERT_EQ(id, CommandId::MoveUp);
+	ASSERT_TRUE(KLookupCtrlCommand('f', id));
+	ASSERT_EQ(id, CommandId::MoveRight);
+	ASSERT_TRUE(KLookupCtrlCommand('b', id));
+	ASSERT_EQ(id, CommandId::MoveLeft);
+
+	ASSERT_TRUE(KLookupCtrlCommand('w', id));
+	ASSERT_EQ(id, CommandId::KillRegion);
+	ASSERT_TRUE(KLookupCtrlCommand('y', id));
+	ASSERT_EQ(id, CommandId::Yank);
+
+	ASSERT_EQ(KLookupCtrlCommand('z', id), false);
+}
+
+
+TEST (KKeymap_EscChords_CanonicalChords)
+{
+	CommandId id{};
+
+	// From docs/ke.md (ESC bindings)
+	ASSERT_TRUE(KLookupEscCommand('b', id));
+	ASSERT_EQ(id, CommandId::WordPrev);
+	ASSERT_TRUE(KLookupEscCommand('f', id));
+	ASSERT_EQ(id, CommandId::WordNext);
+	ASSERT_TRUE(KLookupEscCommand('d', id));
+	ASSERT_EQ(id, CommandId::DeleteWordNext);
+	ASSERT_TRUE(KLookupEscCommand('q', id));
+	ASSERT_EQ(id, CommandId::ReflowParagraph);
+	ASSERT_TRUE(KLookupEscCommand('w', id));
+	ASSERT_EQ(id, CommandId::CopyRegion);
+
+	// ESC BACKSPACE
+	ASSERT_TRUE(KLookupEscCommand(KEY_BACKSPACE, id));
+	ASSERT_EQ(id, CommandId::DeleteWordPrev);
+
+	ASSERT_EQ(KLookupEscCommand('z', id), false);
+}

tests/test_piece_table.cc

@@ -1,8 +1,41 @@
 #include "Test.h"
 #include "PieceTable.h"
+#include <algorithm>
+#include <array>
+#include <random>
 #include <string>
+#include <vector>
 
-TEST(PieceTable_Insert_Delete_LineCount) {
+
+static std::vector<std::size_t>
+LineStartsFor(const std::string &s)
+{
+	std::vector<std::size_t> starts;
+	starts.push_back(0);
+	for (std::size_t i = 0; i < s.size(); i++) {
+		if (s[i] == '\n')
+			starts.push_back(i + 1);
+	}
+	return starts;
+}
+
+
+static std::string
+LineContentFor(const std::string &s, std::size_t line_num)
+{
+	auto starts = LineStartsFor(s);
+	if (starts.empty() || line_num >= starts.size())
+		return std::string();
+	std::size_t start = starts[line_num];
+	std::size_t end   = (line_num + 1 < starts.size()) ? starts[line_num + 1] : s.size();
+	if (end > start && s[end - 1] == '\n')
+		end -= 1;
+	return s.substr(start, end - start);
+}
+
+
+TEST (PieceTable_Insert_Delete_LineCount)
+{
 	PieceTable pt;
 	// start empty
 	ASSERT_EQ(pt.Size(), (std::size_t) 0);
@@ -27,7 +60,9 @@ TEST(PieceTable_Insert_Delete_LineCount) {
 	ASSERT_EQ(pt.GetLine(1), std::string("xyz"));
 }
 
-TEST(PieceTable_LineCol_Conversions) {
+
+TEST (PieceTable_LineCol_Conversions)
+{
 	PieceTable pt;
 	std::string s = "hello\nworld\n"; // two lines with trailing NL
 	pt.Insert(0, s.data(), s.size());
@@ -47,3 +82,100 @@ TEST(PieceTable_LineCol_Conversions) {
 	ASSERT_EQ(lc1.first, (std::size_t) 1);
 	ASSERT_EQ(lc1.second, (std::size_t) 0);
 }
+
+
+TEST (PieceTable_ReferenceModel_RandomEdits_Deterministic)
+{
+	PieceTable pt;
+	std::string model;
+
+	std::mt19937 rng(0xC0FFEEu);
+	const std::vector<std::string> corpus = {
+		"a",
+		"b",
+		"c",
+		"xyz",
+		"123",
+		"\n",
+		"!\n",
+		"foo\nbar",
+		"end\n",
+	};
+
+	auto check_invariants = [&](const char *where) {
+		(void) where;
+		ASSERT_EQ(pt.Size(), model.size());
+		ASSERT_EQ(pt.GetRange(0, pt.Size()), model);
+
+		auto starts = LineStartsFor(model);
+		ASSERT_EQ(pt.LineCount(), starts.size());
+
+		// Spot-check a few line ranges and contents.
+		std::size_t last                             = starts.empty() ? (std::size_t) 0 : (starts.size() - 1);
+		std::size_t mid                              = (starts.size() > 2) ? (std::size_t) 1 : last;
+		const std::array<std::size_t, 3> probe_lines = {(std::size_t) 0, last, mid};
+		for (auto line: probe_lines) {
+			if (starts.empty())
+				break;
+			if (line >= starts.size())
+				continue;
+			std::size_t exp_start = starts[line];
+			std::size_t exp_end   = (line + 1 < starts.size()) ? starts[line + 1] : model.size();
+			auto r                = pt.GetLineRange(line);
+			ASSERT_EQ(r.first, exp_start);
+			ASSERT_EQ(r.second, exp_end);
+			ASSERT_EQ(pt.GetLine(line), LineContentFor(model, line));
+		}
+
+		// Round-trips for a few offsets.
+		const std::vector<std::size_t> probe_offsets = {
+			0,
+			model.size() / 2,
+			model.size(),
+		};
+		for (auto off: probe_offsets) {
+			auto lc   = pt.ByteOffsetToLineCol(off);
+			auto back = pt.LineColToByteOffset(lc.first, lc.second);
+			ASSERT_EQ(back, off);
+		}
+	};
+
+	check_invariants("initial");
+
+	for (int step = 0; step < 250; step++) {
+		bool do_insert = model.empty() || ((rng() % 3u) != 0u); // bias toward insert
+		if (do_insert) {
+			const std::string &ins = corpus[rng() % corpus.size()];
+			std::size_t pos        = model.empty() ? 0 : (rng() % (model.size() + 1));
+			pt.Insert(pos, ins.data(), ins.size());
+			model.insert(pos, ins);
+		} else {
+			std::size_t pos = rng() % model.size();
+			std::size_t max = std::min<std::size_t>(8, model.size() - pos);
+			std::size_t len = 1 + (rng() % max);
+			pt.Delete(pos, len);
+			model.erase(pos, len);
+		}
+
+		// Also validate GetRange on a small random window when non-empty.
+		if (!model.empty()) {
+			std::size_t off = rng() % model.size();
+			std::size_t max = std::min<std::size_t>(16, model.size() - off);
+			std::size_t len = 1 + (rng() % max);
+			ASSERT_EQ(pt.GetRange(off, len), model.substr(off, len));
+		}
+
+		check_invariants("step");
+	}
+
+	// Full line-by-line range verification at the end.
+	auto starts = LineStartsFor(model);
+	for (std::size_t line = 0; line < starts.size(); line++) {
+		std::size_t exp_start = starts[line];
+		std::size_t exp_end   = (line + 1 < starts.size()) ? starts[line + 1] : model.size();
+		auto r                = pt.GetLineRange(line);
+		ASSERT_EQ(r.first, exp_start);
+		ASSERT_EQ(r.second, exp_end);
+		ASSERT_EQ(pt.GetLine(line), LineContentFor(model, line));
+	}
+}

tests/test_reflow_paragraph.cc

@@ -0,0 +1,102 @@
+#include "Test.h"
+
+#include "Buffer.h"
+#include "Command.h"
+#include "Editor.h"
+
+#include <iostream>
+#include <string>
+
+
+static std::string
+to_string_rows(const Buffer &buf)
+{
+	std::string out;
+	for (const auto &r: buf.Rows()) {
+		out += static_cast<std::string>(r);
+		out.push_back('\n');
+	}
+	return out;
+}
+
+
+TEST (ReflowParagraph_NumberedList_HangingIndent)
+{
+	InstallDefaultCommands();
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+
+	Buffer b;
+	// Two list items in one paragraph (no blank lines).
+	// Second line of each item already uses a hanging indent.
+	const std::string initial =
+		"1. one two three four five six seven eight nine ten eleven\n"
+		"   twelve thirteen fourteen\n"
+		"10. alpha beta gamma delta epsilon zeta eta theta iota kappa lambda\n"
+		"    mu nu xi omicron\n";
+	b.insert_text(0, 0, initial);
+	// Put cursor on first item
+	b.SetCursor(0, 0);
+	ed.AddBuffer(std::move(b));
+
+	Buffer *buf = ed.CurrentBuffer();
+	ASSERT_TRUE(buf != nullptr);
+
+	const int width = 25;
+	ASSERT_TRUE(Execute(ed, std::string("reflow-paragraph"), std::string(), width));
+
+	const auto &rows = buf->Rows();
+	ASSERT_TRUE(!rows.empty());
+	const std::string dump = to_string_rows(*buf);
+
+	// Find the start of the second item.
+	bool any_too_long  = false;
+	std::size_t idx_10 = rows.size();
+	for (std::size_t i = 0; i < rows.size(); ++i) {
+		const std::string line = static_cast<std::string>(rows[i]);
+		if (static_cast<int>(line.size()) > width)
+			any_too_long = true;
+		if (line.rfind("10. ", 0) == 0) {
+			idx_10 = i;
+			break;
+		}
+	}
+	ASSERT_TRUE(idx_10 < rows.size());
+	if (any_too_long) {
+		std::cerr << "Reflow produced a line longer than width=" << width << "\n";
+		std::cerr << to_string_rows(*buf) << "\n";
+	}
+	EXPECT_TRUE(!any_too_long);
+
+	// Item 1: first line has "1. ", continuation lines have 3 spaces.
+	for (std::size_t i = 0; i < idx_10; ++i) {
+		const std::string line = static_cast<std::string>(rows[i]);
+		if (i == 0) {
+			ASSERT_TRUE(line.rfind("1. ", 0) == 0);
+		} else {
+			ASSERT_TRUE(line.rfind("   ", 0) == 0);
+			ASSERT_TRUE(line.rfind("1. ", 0) != 0);
+		}
+	}
+
+	// Item 10: first line has "10. ", continuation lines have 4 spaces.
+	ASSERT_TRUE(static_cast<std::string>(rows[idx_10]).rfind("10. ", 0) == 0);
+	bool bad_10 = false;
+	for (std::size_t i = idx_10 + 1; i < rows.size(); ++i) {
+		const std::string line = static_cast<std::string>(rows[i]);
+		if (line.empty())
+			break; // paragraph terminator / trailing empty line
+		if (line.rfind("    ", 0) != 0)
+			bad_10 = true;
+		if (line.rfind("10. ", 0) == 0)
+			bad_10 = true;
+	}
+	if (bad_10) {
+		std::cerr << "Unexpected prefix in reflow output:\n" << dump << "\n";
+	}
+	ASSERT_TRUE(!bad_10);
+
+	// Debug helper if something goes wrong (kept as a string for easy inspection).
+	EXPECT_TRUE(!to_string_rows(*buf).empty());
+}

tests/test_search_replace_flow.cc

@@ -0,0 +1,129 @@
+#include "Test.h"
+
+#include "tests/TestHarness.h"
+
+using ktet::TestHarness;
+
+// These tests intentionally drive the prompt-based search/replace UI headlessly
+// via `Execute(Editor&, CommandId, ...)` to lock down behavior without ncurses.
+
+TEST (SearchFlow_FindStart_Success_LeavesCursorOnMatch_And_ClearsSearchState)
+{
+	TestHarness h;
+	Editor &ed = h.EditorRef();
+	Buffer &b  = h.Buf();
+
+	b.insert_text(0, 0, "abc def abc");
+	b.SetCursor(0, 0);
+	b.SetOffsets(0, 0);
+
+	// Keep a mark set to ensure search doesn't clobber it.
+	b.SetMark(0, 0);
+	ASSERT_TRUE(b.MarkSet());
+
+	ASSERT_TRUE(h.Exec(CommandId::FindStart));
+	ASSERT_TRUE(ed.PromptActive());
+	ASSERT_EQ(ed.CurrentPromptKind(), Editor::PromptKind::Search);
+	ASSERT_TRUE(ed.SearchActive());
+
+	// Typing into the prompt uses InsertText and should jump to the first match.
+	ASSERT_TRUE(h.Exec(CommandId::InsertText, "def"));
+	ASSERT_EQ(b.Cury(), (std::size_t) 0);
+	ASSERT_EQ(b.Curx(), (std::size_t) 4);
+
+	// Enter (Newline) accepts the prompt and ends incremental search.
+	ASSERT_TRUE(h.Exec(CommandId::Newline));
+	ASSERT_TRUE(!ed.PromptActive());
+	ASSERT_TRUE(!ed.SearchActive());
+	ASSERT_TRUE(b.MarkSet());
+}
+
+
+TEST (SearchFlow_FindStart_NotFound_RestoresOrigin_And_ClearsSearchState)
+{
+	TestHarness h;
+	Editor &ed = h.EditorRef();
+	Buffer &b  = h.Buf();
+
+	b.insert_text(0, 0, "hello world\nsecond line\n");
+	b.SetCursor(3, 0);
+	b.SetOffsets(1, 2);
+
+	const std::size_t ox   = b.Curx();
+	const std::size_t oy   = b.Cury();
+	const std::size_t orow = b.Rowoffs();
+	const std::size_t ocol = b.Coloffs();
+
+	ASSERT_TRUE(h.Exec(CommandId::FindStart));
+	ASSERT_TRUE(ed.PromptActive());
+	ASSERT_TRUE(ed.SearchActive());
+
+	// Not-found should restore cursor/viewport to the saved origin while still in prompt.
+	ASSERT_TRUE(h.Exec(CommandId::InsertText, "zzzz"));
+	ASSERT_EQ(b.Curx(), ox);
+	ASSERT_EQ(b.Cury(), oy);
+	ASSERT_EQ(b.Rowoffs(), orow);
+	ASSERT_EQ(b.Coloffs(), ocol);
+
+	ASSERT_TRUE(h.Exec(CommandId::Newline));
+	ASSERT_TRUE(!ed.PromptActive());
+	ASSERT_TRUE(!ed.SearchActive());
+}
+
+
+TEST (SearchFlow_SearchReplace_EmptyFind_DoesNotMutateBuffer_And_ClearsState)
+{
+	TestHarness h;
+	Editor &ed = h.EditorRef();
+	Buffer &b  = h.Buf();
+
+	b.insert_text(0, 0, "abc abc\n");
+	b.SetCursor(0, 0);
+
+	const std::string before = h.Text();
+
+	ASSERT_TRUE(h.Exec(CommandId::SearchReplace));
+	ASSERT_TRUE(ed.PromptActive());
+	ASSERT_EQ(ed.CurrentPromptKind(), Editor::PromptKind::ReplaceFind);
+
+	// Accept empty find -> proceed to ReplaceWith.
+	ASSERT_TRUE(h.Exec(CommandId::Newline));
+	ASSERT_TRUE(ed.PromptActive());
+	ASSERT_EQ(ed.CurrentPromptKind(), Editor::PromptKind::ReplaceWith);
+
+	// Provide replacement and accept -> should cancel due to empty find.
+	ASSERT_TRUE(h.Exec(CommandId::InsertText, "X"));
+	ASSERT_TRUE(h.Exec(CommandId::Newline));
+
+	ASSERT_TRUE(!ed.PromptActive());
+	ASSERT_TRUE(!ed.SearchActive());
+	ASSERT_EQ(h.Text(), before);
+}
+
+
+TEST (SearchFlow_RegexFind_InvalidPattern_FailsSafely_And_ClearsStateOnEnter)
+{
+	TestHarness h;
+	Editor &ed = h.EditorRef();
+	Buffer &b  = h.Buf();
+
+	b.insert_text(0, 0, "abc\ndef\n");
+	b.SetCursor(1, 0);
+	b.SetOffsets(0, 0);
+
+	const std::size_t ox = b.Curx();
+	const std::size_t oy = b.Cury();
+
+	ASSERT_TRUE(h.Exec(CommandId::RegexFindStart));
+	ASSERT_TRUE(ed.PromptActive());
+	ASSERT_EQ(ed.CurrentPromptKind(), Editor::PromptKind::RegexSearch);
+
+	// Invalid regex should not crash; cursor should remain at origin due to no matches.
+	ASSERT_TRUE(h.Exec(CommandId::InsertText, "("));
+	ASSERT_EQ(b.Curx(), ox);
+	ASSERT_EQ(b.Cury(), oy);
+
+	ASSERT_TRUE(h.Exec(CommandId::Newline));
+	ASSERT_TRUE(!ed.PromptActive());
+	ASSERT_TRUE(!ed.SearchActive());
+}

tests/test_swap_recorder.cc

@@ -0,0 +1,104 @@
+#include "Test.h"
+
+#include "Buffer.h"
+#include "SwapRecorder.h"
+
+#include <string>
+#include <vector>
+
+namespace {
+struct SwapEvent {
+	enum class Type {
+		Insert,
+		Delete,
+	};
+
+	Type type;
+	int row;
+	int col;
+	std::string bytes;
+	std::size_t len = 0;
+};
+
+class FakeSwapRecorder final : public kte::SwapRecorder {
+public:
+	std::vector<SwapEvent> events;
+
+
+	void OnInsert(int row, int col, std::string_view bytes) override
+	{
+		SwapEvent e;
+		e.type  = SwapEvent::Type::Insert;
+		e.row   = row;
+		e.col   = col;
+		e.bytes = std::string(bytes);
+		e.len   = 0;
+		events.push_back(std::move(e));
+	}
+
+
+	void OnDelete(int row, int col, std::size_t len) override
+	{
+		SwapEvent e;
+		e.type = SwapEvent::Type::Delete;
+		e.row  = row;
+		e.col  = col;
+		e.len  = len;
+		events.push_back(std::move(e));
+	}
+};
+} // namespace
+
+
+TEST (SwapRecorder_InsertABC)
+{
+	Buffer b;
+	FakeSwapRecorder rec;
+	b.SetSwapRecorder(&rec);
+
+	b.insert_text(0, 0, std::string_view("abc"));
+
+	ASSERT_EQ(rec.events.size(), (std::size_t) 1);
+	ASSERT_TRUE(rec.events[0].type == SwapEvent::Type::Insert);
+	ASSERT_EQ(rec.events[0].row, 0);
+	ASSERT_EQ(rec.events[0].col, 0);
+	ASSERT_EQ(rec.events[0].bytes, std::string("abc"));
+}
+
+
+TEST (SwapRecorder_InsertNewline)
+{
+	Buffer b;
+	FakeSwapRecorder rec;
+	b.SetSwapRecorder(&rec);
+
+	b.split_line(0, 0);
+
+	ASSERT_EQ(rec.events.size(), (std::size_t) 1);
+	ASSERT_TRUE(rec.events[0].type == SwapEvent::Type::Insert);
+	ASSERT_EQ(rec.events[0].row, 0);
+	ASSERT_EQ(rec.events[0].col, 0);
+	ASSERT_EQ(rec.events[0].bytes, std::string("\n"));
+}
+
+
+TEST (SwapRecorder_DeleteSpanningNewline)
+{
+	Buffer b;
+	// Prepare content without a recorder (should be no-op)
+	b.insert_text(0, 0, std::string_view("ab"));
+	b.split_line(0, 2);
+	b.insert_text(1, 0, std::string_view("cd"));
+
+	FakeSwapRecorder rec;
+	b.SetSwapRecorder(&rec);
+
+	// Delete "b\n c" (3 bytes) starting at row 0, col 1.
+	b.delete_text(0, 1, 3);
+
+	ASSERT_EQ(rec.events.size(), (std::size_t) 1);
+	ASSERT_TRUE(rec.events[0].type == SwapEvent::Type::Delete);
+	ASSERT_EQ(rec.events[0].row, 0);
+	ASSERT_EQ(rec.events[0].col, 1);
+	ASSERT_EQ(rec.events[0].len, (std::size_t) 3);
+}

tests/test_swap_replay.cc

@@ -0,0 +1,114 @@
+#include "Test.h"
+
+#include "Buffer.h"
+#include "Swap.h"
+
+#include <cstdio>
+#include <fstream>
+#include <string>
+
+
+static void
+write_file_bytes(const std::string &path, const std::string &bytes)
+{
+	std::ofstream out(path, std::ios::binary | std::ios::trunc);
+	out.write(bytes.data(), (std::streamsize) bytes.size());
+}
+
+
+static std::string
+read_file_bytes(const std::string &path)
+{
+	std::ifstream in(path, std::ios::binary);
+	return std::string((std::istreambuf_iterator<char>(in)), std::istreambuf_iterator<char>());
+}
+
+
+static std::string
+buffer_bytes_via_views(const Buffer &b)
+{
+	const auto &rows = b.Rows();
+	std::string out;
+	for (std::size_t i = 0; i < rows.size(); i++) {
+		auto v = b.GetLineView(i);
+		out.append(v.data(), v.size());
+	}
+	return out;
+}
+
+
+TEST (SwapReplay_RecordFlushReopenReplay_ExactBytesMatch)
+{
+	const std::string path = "./.kte_ut_swap_replay_1.txt";
+	std::remove(path.c_str());
+	write_file_bytes(path, "base\nline2\n");
+
+	Buffer b;
+	std::string err;
+	ASSERT_TRUE(b.OpenFromFile(path, err));
+
+	kte::SwapManager sm;
+	sm.Attach(&b);
+	b.SetSwapRecorder(sm.RecorderFor(&b));
+
+	// Edits (no save): swap should capture these.
+	b.insert_text(0, 0, std::string("X")); // Xbase\nline2\n
+	b.delete_text(1, 1, 2); // delete "in" from "line2"
+	b.split_line(0, 3); // Xba\nse...
+	b.join_lines(0); // join back
+	b.insert_text(1, 0, std::string("ZZ")); // insert at start of line2
+	b.delete_text(0, 0, 1); // delete leading X
+
+	sm.Flush(&b);
+	const std::string swap_path = kte::SwapManager::ComputeSwapPathForTests(b);
+	const std::string expected  = buffer_bytes_via_views(b);
+
+	// Close journal before replaying (for determinism)
+	b.SetSwapRecorder(nullptr);
+	sm.Detach(&b);
+
+	Buffer b2;
+	ASSERT_TRUE(b2.OpenFromFile(path, err));
+	ASSERT_TRUE(kte::SwapManager::ReplayFile(b2, swap_path, err));
+	ASSERT_EQ(buffer_bytes_via_views(b2), expected);
+
+	std::remove(path.c_str());
+	std::remove(swap_path.c_str());
+}
+
+
+TEST (SwapReplay_TruncatedLog_FailsSafely)
+{
+	const std::string path = "./.kte_ut_swap_replay_2.txt";
+	std::remove(path.c_str());
+	write_file_bytes(path, "hello\n");
+
+	Buffer b;
+	std::string err;
+	ASSERT_TRUE(b.OpenFromFile(path, err));
+
+	kte::SwapManager sm;
+	sm.Attach(&b);
+	b.SetSwapRecorder(sm.RecorderFor(&b));
+	b.insert_text(0, 0, std::string("X"));
+	sm.Flush(&b);
+	const std::string swap_path = kte::SwapManager::ComputeSwapPathForTests(b);
+	b.SetSwapRecorder(nullptr);
+	sm.Detach(&b);
+
+	const std::string bytes = read_file_bytes(swap_path);
+	ASSERT_TRUE(bytes.size() > 70); // header + at least one record
+
+	const std::string trunc_path = swap_path + ".trunc";
+	write_file_bytes(trunc_path, bytes.substr(0, bytes.size() - 1));
+
+	Buffer b2;
+	ASSERT_TRUE(b2.OpenFromFile(path, err));
+	std::string rerr;
+	ASSERT_EQ(kte::SwapManager::ReplayFile(b2, trunc_path, rerr), false);
+	ASSERT_EQ(rerr.empty(), false);
+
+	std::remove(path.c_str());
+	std::remove(swap_path.c_str());
+	std::remove(trunc_path.c_str());
+}

tests/test_swap_writer.cc

@@ -0,0 +1,236 @@
+#include "Test.h"
+
+#include "Buffer.h"
+#include "Swap.h"
+
+#include <cstdint>
+#include <filesystem>
+#include <fstream>
+#include <string>
+#include <vector>
+
+#include <cstdlib>
+#include <cstdio>
+#include <unistd.h>
+
+#include <sys/stat.h>
+
+namespace {
+std::vector<std::uint8_t>
+read_all_bytes(const std::string &path)
+{
+	std::ifstream in(path, std::ios::binary);
+	return std::vector<std::uint8_t>((std::istreambuf_iterator<char>(in)), std::istreambuf_iterator<char>());
+}
+
+
+std::uint32_t
+read_le32(const std::uint8_t *p)
+{
+	return (std::uint32_t) p[0] | ((std::uint32_t) p[1] << 8) | ((std::uint32_t) p[2] << 16) |
+	       ((std::uint32_t) p[3] << 24);
+}
+
+
+std::uint64_t
+read_le64(const std::uint8_t *p)
+{
+	std::uint64_t v = 0;
+	for (int i = 7; i >= 0; --i) {
+		v = (v << 8) | p[i];
+	}
+	return v;
+}
+
+
+std::uint32_t
+crc32(const std::uint8_t *data, std::size_t len, std::uint32_t seed = 0)
+{
+	static std::uint32_t table[256];
+	static bool inited = false;
+	if (!inited) {
+		for (std::uint32_t i = 0; i < 256; ++i) {
+			std::uint32_t c = i;
+			for (int j = 0; j < 8; ++j)
+				c = (c & 1) ? (0xEDB88320u ^ (c >> 1)) : (c >> 1);
+			table[i] = c;
+		}
+		inited = true;
+	}
+	std::uint32_t c = ~seed;
+	for (std::size_t i = 0; i < len; ++i)
+		c = table[(c ^ data[i]) & 0xFFu] ^ (c >> 8);
+	return ~c;
+}
+} // namespace
+
+
+TEST (SwapWriter_Header_Records_And_CRC)
+{
+	const std::filesystem::path xdg_root = std::filesystem::temp_directory_path() /
+	                                       (std::string("kte_ut_xdg_state_") + std::to_string((int) ::getpid()));
+	std::filesystem::remove_all(xdg_root);
+
+	const char *old_xdg = std::getenv("XDG_STATE_HOME");
+	setenv("XDG_STATE_HOME", xdg_root.string().c_str(), 1);
+
+	const std::string path = (xdg_root / "work" / "kte_ut_swap_writer.txt").string();
+	std::filesystem::create_directories((xdg_root / "work"));
+
+	// Clean up from prior runs
+	std::remove(path.c_str());
+
+	// Ensure file exists so buffer is file-backed
+	{
+		std::ofstream out(path, std::ios::binary);
+		out << "";
+	}
+
+	Buffer b;
+	std::string err;
+	ASSERT_TRUE(b.OpenFromFile(path, err));
+	ASSERT_TRUE(err.empty());
+	ASSERT_TRUE(b.IsFileBacked());
+
+	kte::SwapManager sm;
+	sm.Attach(&b);
+	b.SetSwapRecorder(sm.RecorderFor(&b));
+	const std::string swp = kte::SwapManager::ComputeSwapPathForTests(b);
+	std::remove(swp.c_str());
+
+	// Emit one INS and one DEL
+	b.insert_text(0, 0, std::string_view("abc"));
+	b.delete_text(0, 1, 1);
+
+	// Ensure all records are written before reading
+	sm.Flush(&b);
+	sm.Detach(&b);
+	b.SetSwapRecorder(nullptr);
+
+	ASSERT_TRUE(std::filesystem::exists(swp));
+
+	// Verify permissions 0600
+	struct stat st{};
+	ASSERT_TRUE(::stat(swp.c_str(), &st) == 0);
+	ASSERT_EQ((st.st_mode & 0777), 0600);
+
+	const std::vector<std::uint8_t> bytes = read_all_bytes(swp);
+	ASSERT_TRUE(bytes.size() >= 64);
+
+	// Header
+	static const std::uint8_t magic[8] = {'K', 'T', 'E', '_', 'S', 'W', 'P', '\0'};
+	for (int i = 0; i < 8; ++i)
+		ASSERT_EQ(bytes[(std::size_t) i], magic[i]);
+	ASSERT_EQ(read_le32(bytes.data() + 8), (std::uint32_t) 1);
+	// flags currently 0
+	ASSERT_EQ(read_le32(bytes.data() + 12), (std::uint32_t) 0);
+	ASSERT_TRUE(read_le64(bytes.data() + 16) != 0);
+
+	// Records
+	std::vector<std::uint8_t> types;
+	std::size_t off = 64;
+	while (off < bytes.size()) {
+		ASSERT_TRUE(bytes.size() - off >= 8); // at least header+crc
+		const std::uint8_t type = bytes[off + 0];
+		const std::uint32_t len = (std::uint32_t) bytes[off + 1] | ((std::uint32_t) bytes[off + 2] << 8) |
+		                          ((std::uint32_t) bytes[off + 3] << 16);
+		const std::size_t payload_off = off + 4;
+		const std::size_t crc_off     = payload_off + len;
+		ASSERT_TRUE(crc_off + 4 <= bytes.size());
+
+		const std::uint32_t got_crc = read_le32(bytes.data() + crc_off);
+		std::uint32_t c             = 0;
+		c                           = crc32(bytes.data() + off, 4, c);
+		c                           = crc32(bytes.data() + payload_off, len, c);
+		ASSERT_EQ(got_crc, c);
+
+		types.push_back(type);
+		off = crc_off + 4;
+	}
+
+	ASSERT_EQ(types.size(), (std::size_t) 2);
+	ASSERT_EQ(types[0], (std::uint8_t) kte::SwapRecType::INS);
+	ASSERT_EQ(types[1], (std::uint8_t) kte::SwapRecType::DEL);
+
+	std::remove(path.c_str());
+	std::remove(swp.c_str());
+	if (old_xdg) {
+		setenv("XDG_STATE_HOME", old_xdg, 1);
+	} else {
+		unsetenv("XDG_STATE_HOME");
+	}
+	std::filesystem::remove_all(xdg_root);
+}
+
+
+TEST (SwapWriter_NoStomp_SameBasename)
+{
+	const std::filesystem::path xdg_root = std::filesystem::temp_directory_path() /
+	                                       (std::string("kte_ut_xdg_state_nostomp_") + std::to_string(
+		                                        (int) ::getpid()));
+	std::filesystem::remove_all(xdg_root);
+	std::filesystem::create_directories(xdg_root);
+
+	const char *old_xdg = std::getenv("XDG_STATE_HOME");
+	setenv("XDG_STATE_HOME", xdg_root.string().c_str(), 1);
+
+	const std::filesystem::path d1 = xdg_root / "p1";
+	const std::filesystem::path d2 = xdg_root / "p2";
+	std::filesystem::create_directories(d1);
+	std::filesystem::create_directories(d2);
+	const std::filesystem::path f1 = d1 / "same.txt";
+	const std::filesystem::path f2 = d2 / "same.txt";
+
+	{
+		std::ofstream out(f1.string(), std::ios::binary);
+		out << "";
+	}
+	{
+		std::ofstream out(f2.string(), std::ios::binary);
+		out << "";
+	}
+
+	Buffer b1;
+	Buffer b2;
+	std::string err;
+	ASSERT_TRUE(b1.OpenFromFile(f1.string(), err));
+	ASSERT_TRUE(err.empty());
+	ASSERT_TRUE(b2.OpenFromFile(f2.string(), err));
+	ASSERT_TRUE(err.empty());
+
+	const std::string swp1 = kte::SwapManager::ComputeSwapPathForTests(b1);
+	const std::string swp2 = kte::SwapManager::ComputeSwapPathForTests(b2);
+	ASSERT_TRUE(swp1 != swp2);
+
+	// Actually write to both to ensure one doesn't clobber the other.
+	kte::SwapManager sm;
+	sm.Attach(&b1);
+	sm.Attach(&b2);
+	b1.SetSwapRecorder(sm.RecorderFor(&b1));
+	b2.SetSwapRecorder(sm.RecorderFor(&b2));
+
+	b1.insert_text(0, 0, std::string_view("one"));
+	b2.insert_text(0, 0, std::string_view("two"));
+	sm.Flush();
+
+	ASSERT_TRUE(std::filesystem::exists(swp1));
+	ASSERT_TRUE(std::filesystem::exists(swp2));
+	ASSERT_TRUE(std::filesystem::file_size(swp1) >= 64);
+	ASSERT_TRUE(std::filesystem::file_size(swp2) >= 64);
+
+	sm.Detach(&b1);
+	sm.Detach(&b2);
+	b1.SetSwapRecorder(nullptr);
+	b2.SetSwapRecorder(nullptr);
+
+	std::remove(swp1.c_str());
+	std::remove(swp2.c_str());
+	std::remove(f1.string().c_str());
+	std::remove(f2.string().c_str());
+	if (old_xdg) {
+		setenv("XDG_STATE_HOME", old_xdg, 1);
+	} else {
+		unsetenv("XDG_STATE_HOME");
+	}
+	std::filesystem::remove_all(xdg_root);
+}

tests/test_undo.cc

@@ -0,0 +1,940 @@
+#include "Test.h"
+#include "Buffer.h"
+
+#include "Command.h"
+#include "Editor.h"
+
+#include <cstddef>
+#include <random>
+
+#if defined(KTE_TESTS)
+#include <unordered_set>
+
+static void
+validate_undo_subtree(const UndoNode *node, const UndoNode *expected_parent,
+                      std::unordered_set<const UndoNode *> &seen)
+{
+	ASSERT_TRUE(node != nullptr);
+	ASSERT_TRUE(seen.find(node) == seen.end());
+	seen.insert(node);
+	ASSERT_TRUE(node->parent == expected_parent);
+
+	// Validate each redo branch under this node.
+	for (const UndoNode *ch = node->child; ch != nullptr; ch = ch->next) {
+		validate_undo_subtree(ch, node, seen);
+	}
+}
+
+
+static void
+validate_undo_tree(const UndoSystem &u)
+{
+	const UndoTree &t = u.TreeForTests();
+
+	std::unordered_set<const UndoNode *> seen;
+	for (const UndoNode *root = t.root; root != nullptr; root = root->next) {
+		validate_undo_subtree(root, nullptr, seen);
+	}
+
+	// current/saved must either be null or be reachable from some root.
+	if (t.current)
+		ASSERT_TRUE(seen.find(t.current) != seen.end());
+	if (t.saved)
+		ASSERT_TRUE(seen.find(t.saved) != seen.end());
+
+	// pending is detached (not part of the committed tree).
+	if (t.pending) {
+		ASSERT_TRUE(seen.find(t.pending) == seen.end());
+		ASSERT_TRUE(t.pending->parent == nullptr);
+		ASSERT_TRUE(t.pending->child == nullptr);
+		ASSERT_TRUE(t.pending->next == nullptr);
+	}
+}
+#endif
+
+
+TEST (Undo_InsertRun_Coalesces)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Simulate two separate "typed" insert commands without committing in between.
+	b.SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 0, std::string_view("h"));
+	u->Append('h');
+	b.SetCursor(1, 0);
+
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("i"));
+	u->Append('i');
+	b.SetCursor(2, 0);
+
+	u->commit();
+	ASSERT_EQ(b.Rows().size(), (std::size_t) 1);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("hi"));
+
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string(""));
+}
+
+
+TEST (Undo_BackspaceRun_Coalesces)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Seed content.
+	b.insert_text(0, 0, std::string_view("abc"));
+	b.SetCursor(3, 0);
+	u->mark_saved();
+
+	// Simulate two backspaces: delete 'c' then 'b'.
+	{
+		const auto &rows = b.Rows();
+		char deleted     = rows[0][2];
+		b.delete_text(0, 2, 1);
+		b.SetCursor(2, 0);
+		u->Begin(UndoType::Delete);
+		u->Append(deleted);
+	}
+	{
+		const auto &rows = b.Rows();
+		char deleted     = rows[0][1];
+		b.delete_text(0, 1, 1);
+		b.SetCursor(1, 0);
+		u->Begin(UndoType::Delete);
+		u->Append(deleted);
+	}
+
+	u->commit();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+
+	// One undo should restore both characters.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("abc"));
+}
+
+
+TEST (Undo_Branching_RedoPreservedAfterNewEdit)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	b.SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 0, std::string_view("a"));
+	u->Append('a');
+	b.SetCursor(1, 0);
+	u->commit();
+
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("b"));
+	u->Append('b');
+	b.SetCursor(2, 0);
+	u->commit();
+
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+
+	// New edit after undo creates a new branch; the old redo should remain as an alternate branch.
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("c"));
+	u->Append('c');
+	b.SetCursor(2, 0);
+	u->commit();
+
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ac"));
+
+	// No further redo from the tip.
+	u->redo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ac"));
+
+	// Undo back to the branch point and redo the original branch.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+	u->redo(1);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+}
+
+
+TEST (Undo_DirtyFlag_MarkSavedAndUndoRedo)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	u->mark_saved();
+	ASSERT_TRUE(!b.Dirty());
+
+	b.SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 0, std::string_view("x"));
+	u->Append('x');
+	b.SetCursor(1, 0);
+	u->commit();
+
+	ASSERT_TRUE(b.Dirty());
+	u->undo();
+	ASSERT_TRUE(!b.Dirty());
+	u->redo();
+	ASSERT_TRUE(b.Dirty());
+}
+
+
+TEST (Undo_Newline_UndoRedo_SplitJoin)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Seed a single line and split it.
+	b.insert_text(0, 0, std::string_view("hello"));
+	b.SetCursor(2, 0); // split after "he"
+	u->Begin(UndoType::Newline);
+	b.split_line(0, 2);
+	u->commit();
+
+	ASSERT_EQ(b.Rows().size(), (std::size_t) 2);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("he"));
+	ASSERT_EQ(std::string(b.Rows()[1]), std::string("llo"));
+
+	// Undo should join the lines back.
+	u->undo();
+	ASSERT_EQ(b.Rows().size(), (std::size_t) 1);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("hello"));
+
+	// Redo should split again at the same point.
+	u->redo();
+	ASSERT_EQ(b.Rows().size(), (std::size_t) 2);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("he"));
+	ASSERT_EQ(std::string(b.Rows()[1]), std::string("llo"));
+}
+
+
+TEST (Undo_DeleteKeyRun_Coalesces)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Seed content: delete-key semantics keep cursor at the same column.
+	b.insert_text(0, 0, std::string_view("abcd"));
+	b.SetCursor(1, 0); // on 'b'
+
+	// Delete 'b'
+	{
+		const auto &rows = b.Rows();
+		char deleted     = rows[0][1];
+		u->Begin(UndoType::Delete);
+		b.delete_text(0, 1, 1);
+		u->Append(deleted);
+		b.SetCursor(1, 0);
+	}
+	// Delete next char (was 'c', now at same col=1)
+	{
+		const auto &rows = b.Rows();
+		char deleted     = rows[0][1];
+		u->Begin(UndoType::Delete);
+		b.delete_text(0, 1, 1);
+		u->Append(deleted);
+		b.SetCursor(1, 0);
+	}
+
+	u->commit();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ad"));
+
+	// One undo should restore both deleted characters.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("abcd"));
+}
+
+
+TEST (Undo_UndoPastFirstEdit_RedoFromPreFirstEdit)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Commit two separate insert edits.
+	b.SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 0, std::string_view("a"));
+	u->Append('a');
+	b.SetCursor(1, 0);
+	u->commit();
+
+	b.SetCursor(1, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("b"));
+	u->Append('b');
+	b.SetCursor(2, 0);
+	u->commit();
+
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+
+	// Undo twice: we should reach the pre-first-edit state.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string(""));
+
+	// Redo twice should restore both edits.
+	u->redo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+	u->redo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+}
+
+
+TEST (Undo_NewEditFromPreFirstEdit_PreservesOldHistoryAsAlternateRootBranch)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Build up two edits.
+	b.SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 0, std::string_view("a"));
+	u->Append('a');
+	b.SetCursor(1, 0);
+	u->commit();
+
+	b.SetCursor(1, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("b"));
+	u->Append('b');
+	b.SetCursor(2, 0);
+	u->commit();
+
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+
+	// Undo past first edit so current becomes null.
+	u->undo();
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string(""));
+
+	// Commit a new edit from the pre-first-edit state.
+	b.SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 0, std::string_view("x"));
+	u->Append('x');
+	b.SetCursor(1, 0);
+	u->commit();
+
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("x"));
+
+	// From the tip, no further redo.
+	u->redo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("x"));
+
+	// Undo back to pre-first-edit and select the older root branch.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string(""));
+	u->redo(1);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+}
+
+
+TEST (Undo_MultiLineDelete_ConsumesNewline_UndoRestores)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Create two lines. PieceTable treats '\n' between logical lines.
+	b.insert_text(0, 0, std::string_view("ab\ncd"));
+	ASSERT_EQ(b.Rows().size(), (std::size_t) 2);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+	ASSERT_EQ(std::string(b.Rows()[1]), std::string("cd"));
+
+	// Delete spanning the newline: delete "b\n" starting at (0,1).
+	b.SetCursor(1, 0);
+	u->Begin(UndoType::Delete);
+	b.delete_text(0, 1, 2);
+	u->Append(std::string_view("b\n"));
+	u->commit();
+
+	ASSERT_EQ(b.Rows().size(), (std::size_t) 1);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("acd"));
+
+	// Undo should restore exact original text/line structure.
+	u->undo();
+	ASSERT_EQ(b.Rows().size(), (std::size_t) 2);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+	ASSERT_EQ(std::string(b.Rows()[1]), std::string("cd"));
+}
+
+
+TEST (Undo_DeleteIndent_UndoRestoresCursorAtText)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Seed 3-line content with indentation on the middle line.
+	b.insert_text(0, 0,
+	              std::string_view("I did a thing\n  and then I edited a thing\nbut there were gaps"));
+	ASSERT_EQ(b.Rows().size(), (std::size_t) 3);
+
+	// Cursor at start of the line (before spaces), then C-d C-d deletes two spaces.
+	b.SetCursor(0, 1);
+	for (int i = 0; i < 2; ++i) {
+		const auto &rows = b.Rows();
+		char deleted     = rows[1][0];
+		ASSERT_EQ(deleted, ' ');
+		u->Begin(UndoType::Delete);
+		b.delete_text(1, 0, 1);
+		u->Append(deleted);
+		b.SetCursor(0, 1); // delete-key keeps col the same
+	}
+	u->commit();
+
+	ASSERT_EQ(std::string(b.Rows()[1]), std::string("and then I edited a thing"));
+	ASSERT_EQ(b.Cury(), (std::size_t) 1);
+	ASSERT_EQ(b.Curx(), (std::size_t) 0);
+
+	// Undo should restore indentation, and keep cursor on the text (at 'a'), not at EOL.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[1]), std::string("  and then I edited a thing"));
+	ASSERT_EQ(b.Cury(), (std::size_t) 1);
+	ASSERT_EQ(b.Curx(), (std::size_t) 2);
+}
+
+
+TEST (Undo_StructuralInvariants_BranchingAndRoots)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Build history: a -> b
+	b.SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 0, std::string_view("a"));
+	u->Append('a');
+	b.SetCursor(1, 0);
+	u->commit();
+
+	b.SetCursor(1, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("b"));
+	u->Append('b');
+	b.SetCursor(2, 0);
+	u->commit();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+
+	// Undo past first edit; now create a new root-level branch x.
+	u->undo();
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string(""));
+
+	b.SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 0, std::string_view("x"));
+	u->Append('x');
+	b.SetCursor(1, 0);
+	u->commit();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("x"));
+
+	// Return to the older root branch.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string(""));
+	u->redo(1);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+
+	// Create a normal branch under 'a'.
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("c"));
+	u->Append('c');
+	b.SetCursor(2, 0);
+	u->commit();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ac"));
+
+	validate_undo_tree(*u);
+}
+
+
+TEST (Undo_BranchSelection_ThreeSiblingsAndHeadPersists)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Root: a
+	b.SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 0, std::string_view("a"));
+	u->Append('a');
+	b.SetCursor(1, 0);
+	u->commit();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+
+	// Branch 1: a->b
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("b"));
+	u->Append('b');
+	b.SetCursor(2, 0);
+	u->commit();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+
+	// Back to branch point.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+
+	// Branch 2: a->c
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("c"));
+	u->Append('c');
+	b.SetCursor(2, 0);
+	u->commit();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ac"));
+
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+
+	// Branch 3: a->d
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("d"));
+	u->Append('d');
+	b.SetCursor(2, 0);
+	u->commit();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ad"));
+
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+
+	// Under 'a', the sibling list should now contain 3 branches.
+	validate_undo_tree(*u);
+
+	// Select the 3rd sibling (branch_index=2) which should be the oldest ("b"), and make it active.
+	u->redo(2);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+
+	// Since we selected "b", redo with default should now follow "b" again.
+	u->redo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+
+	// Select another branch by index and ensure it becomes the new default.
+	u->redo(1);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ad"));
+	u->undo();
+	u->redo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ad"));
+	u->undo();
+
+	// Out-of-range selection should be a no-op.
+	u->redo(99);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+
+	validate_undo_tree(*u);
+}
+
+
+TEST (Undo_Branching_SwitchBetweenTwoRedoBranches_TextAndCursor)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Build A->B.
+	b.SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 0, std::string_view("a"));
+	u->Append('a');
+	b.SetCursor(1, 0);
+	u->commit();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+	ASSERT_EQ(b.Cury(), (std::size_t) 0);
+	ASSERT_EQ(b.Curx(), (std::size_t) 1);
+
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("b"));
+	u->Append('b');
+	b.SetCursor(2, 0);
+	u->commit();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+	ASSERT_EQ(b.Curx(), (std::size_t) 2);
+
+	// Undo to A.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+	ASSERT_EQ(b.Curx(), (std::size_t) 1);
+
+	// Create sibling branch A->C.
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("c"));
+	u->Append('c');
+	b.SetCursor(2, 0);
+	u->commit();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ac"));
+	ASSERT_EQ(b.Curx(), (std::size_t) 2);
+
+	// Back to A.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+	ASSERT_EQ(b.Curx(), (std::size_t) 1);
+
+	// Redo into B as the alternate branch (older sibling), and confirm cursor is consistent.
+	u->redo(1);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+	ASSERT_EQ(b.Curx(), (std::size_t) 2);
+
+	// Both branches remain reachable: undo to A, redo defaults to B (head reordered).
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+	u->redo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+
+	// And the other branch C should still be selectable.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+	u->redo(1);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ac"));
+	ASSERT_EQ(b.Curx(), (std::size_t) 2);
+
+	// After selecting C, default redo from A should now follow C.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+	u->redo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ac"));
+
+	validate_undo_tree(*u);
+}
+
+
+TEST (Undo_Randomized_Deterministic_EditUndoRedoBranchSelect)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	std::mt19937 rng(0xC0FFEEu);
+	std::uniform_int_distribution<int> op(0, 99);
+	std::uniform_int_distribution<int> ch(0, 25);
+
+	const int steps      = 300;
+	const int max_len    = 40;
+	const int max_branch = 4;
+
+	for (int i = 0; i < steps; ++i) {
+		ASSERT_TRUE(!b.Rows().empty());
+		ASSERT_EQ(b.Cury(), (std::size_t) 0);
+		ASSERT_EQ(b.Rows().size(), (std::size_t) 1);
+		ASSERT_TRUE(b.Curx() <= b.Rows()[0].size());
+
+		validate_undo_tree(*u);
+
+		int r           = op(rng);
+		std::string cur = std::string(b.Rows()[0]);
+		int len         = static_cast<int>(cur.size());
+
+		if (r < 40 && len < max_len) {
+			// Insert one char at end as a standalone committed node.
+			char c = static_cast<char>('a' + ch(rng));
+			b.SetCursor(static_cast<std::size_t>(len), 0);
+			u->Begin(UndoType::Insert);
+			b.insert_text(0, len, std::string_view(&c, 1));
+			u->Append(c);
+			b.SetCursor(static_cast<std::size_t>(len + 1), 0);
+			u->commit();
+		} else if (r < 60 && len > 0) {
+			// Backspace at end as a standalone committed node.
+			char deleted = cur[static_cast<std::size_t>(len - 1)];
+			b.delete_text(0, len - 1, 1);
+			b.SetCursor(static_cast<std::size_t>(len - 1), 0);
+			u->Begin(UndoType::Delete);
+			u->Append(deleted);
+			u->commit();
+		} else if (r < 80) {
+			// Undo then redo should round-trip to the exact same node/text/cursor when possible.
+			const UndoNode *before_node = u->TreeForTests().current;
+			const std::string before_text(std::string(b.Rows()[0]));
+			const std::size_t before_x = b.Curx();
+
+			if (before_node) {
+				u->undo();
+				u->redo();
+				ASSERT_TRUE(u->TreeForTests().current == before_node);
+				ASSERT_EQ(std::string(b.Rows()[0]), before_text);
+				ASSERT_EQ(b.Curx(), before_x);
+			} else {
+				// Nothing to undo; just exercise redo/branch-select paths.
+				u->redo();
+			}
+		} else if (r < 90) {
+			u->undo();
+		} else {
+			int idx = static_cast<int>(rng() % static_cast<std::uint32_t>(max_branch));
+			if ((rng() % 8u) == 0u)
+				idx = 99; // intentionally out of range sometimes
+			u->redo(idx);
+		}
+	}
+
+	validate_undo_tree(*u);
+}
+
+
+TEST (Undo_PendingCoalescedRun_UndoCommitsThenUndoes)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Create a coalesced insert run without an explicit commit.
+	b.SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 0, std::string_view("a"));
+	u->Append('a');
+	b.SetCursor(1, 0);
+
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("b"));
+	u->Append('b');
+	b.SetCursor(2, 0);
+
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+
+	// undo() should implicitly commit pending and then undo it as one step.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string(""));
+
+	u->redo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+
+	validate_undo_tree(*u);
+}
+
+
+TEST (Undo_PendingRunAtBranchPoint_UndoThenBranchSelectionStillWorks)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Build a->b.
+	b.SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 0, std::string_view("a"));
+	u->Append('a');
+	b.SetCursor(1, 0);
+	u->commit();
+
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("b"));
+	u->Append('b');
+	b.SetCursor(2, 0);
+	u->commit();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+
+	// Undo to the branch point.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+
+	// Start a pending insert "c" at the branch point, but don't commit.
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("c"));
+	u->Append('c');
+	b.SetCursor(2, 0);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ac"));
+
+	// Undo should seal the pending "c" as a new branch, then undo it, leaving us at "a".
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+
+	// The active redo should now be "c".
+	u->redo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ac"));
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+
+	// Select the older "b" branch.
+	u->redo(1);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+
+	validate_undo_tree(*u);
+}
+
+
+TEST (Undo_SavedNodeOnOtherBranch_DirtyClearsWhenReturning)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Build a->b and mark saved at the tip.
+	b.SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 0, std::string_view("a"));
+	u->Append('a');
+	b.SetCursor(1, 0);
+	u->commit();
+
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("b"));
+	u->Append('b');
+	b.SetCursor(2, 0);
+	u->commit();
+	u->mark_saved();
+	ASSERT_TRUE(!b.Dirty());
+
+	// Move to a different branch.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("c"));
+	u->Append('c');
+	b.SetCursor(2, 0);
+	u->commit();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ac"));
+	ASSERT_TRUE(b.Dirty());
+
+	// Return to the saved node by selecting the older branch.
+	u->undo();
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("a"));
+	u->redo(1);
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("ab"));
+	ASSERT_TRUE(!b.Dirty());
+
+	validate_undo_tree(*u);
+}
+
+
+TEST (Undo_Clear_AfterSaved_ResetsStateSafely)
+{
+	Buffer b;
+	UndoSystem *u = b.Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	b.SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 0, std::string_view("x"));
+	u->Append('x');
+	b.SetCursor(1, 0);
+	u->commit();
+	u->mark_saved();
+	ASSERT_TRUE(!b.Dirty());
+
+	u->Begin(UndoType::Insert);
+	b.insert_text(0, 1, std::string_view("y"));
+	u->Append('y');
+	b.SetCursor(2, 0);
+	u->commit();
+	ASSERT_TRUE(b.Dirty());
+
+	u->clear();
+	ASSERT_TRUE(!b.Dirty());
+	// clear() resets undo history, but does not mutate buffer contents.
+	ASSERT_EQ(std::string(b.Rows()[0]), std::string("xy"));
+
+	validate_undo_tree(*u);
+}
+
+
+TEST (Undo_Command_UndoHonorsRepeatCount)
+{
+	InstallDefaultCommands();
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+
+	Buffer b;
+	ed.AddBuffer(std::move(b));
+	Buffer *buf = ed.CurrentBuffer();
+	ASSERT_TRUE(buf != nullptr);
+	UndoSystem *u = buf->Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Create two committed steps using the undo system directly.
+	buf->SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	buf->insert_text(0, 0, std::string_view("a"));
+	u->Append('a');
+	buf->SetCursor(1, 0);
+	u->commit();
+
+	u->Begin(UndoType::Insert);
+	buf->insert_text(0, 1, std::string_view("b"));
+	u->Append('b');
+	buf->SetCursor(2, 0);
+	u->commit();
+	ASSERT_EQ(std::string(buf->Rows()[0]), std::string("ab"));
+
+	// Undo twice via command repeat count.
+	ed.SetUniversalArg(1, 2);
+	ASSERT_TRUE(Execute(ed, CommandId::Undo));
+	ASSERT_EQ(std::string(buf->Rows()[0]), std::string(""));
+
+	validate_undo_tree(*u);
+}
+
+
+TEST (Undo_Command_RedoCountSelectsBranch)
+{
+	InstallDefaultCommands();
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+
+	Buffer b;
+	ed.AddBuffer(std::move(b));
+	Buffer *buf = ed.CurrentBuffer();
+	ASSERT_TRUE(buf != nullptr);
+	UndoSystem *u = buf->Undo();
+	ASSERT_TRUE(u != nullptr);
+
+	// Build a->b.
+	buf->SetCursor(0, 0);
+	u->Begin(UndoType::Insert);
+	buf->insert_text(0, 0, std::string_view("a"));
+	u->Append('a');
+	buf->SetCursor(1, 0);
+	u->commit();
+
+	u->Begin(UndoType::Insert);
+	buf->insert_text(0, 1, std::string_view("b"));
+	u->Append('b');
+	buf->SetCursor(2, 0);
+	u->commit();
+	ASSERT_EQ(std::string(buf->Rows()[0]), std::string("ab"));
+
+	// Undo to the branch point and create a sibling branch "c".
+	u->undo();
+	ASSERT_EQ(std::string(buf->Rows()[0]), std::string("a"));
+
+	u->Begin(UndoType::Insert);
+	buf->insert_text(0, 1, std::string_view("c"));
+	u->Append('c');
+	buf->SetCursor(2, 0);
+	u->commit();
+	ASSERT_EQ(std::string(buf->Rows()[0]), std::string("ac"));
+
+	// Back to branch point.
+	ASSERT_TRUE(Execute(ed, CommandId::Undo));
+	ASSERT_EQ(std::string(buf->Rows()[0]), std::string("a"));
+
+	// Command redo with count=2 should select branch_index=1 (the older "b" branch).
+	ed.SetUniversalArg(1, 2);
+	ASSERT_TRUE(Execute(ed, CommandId::Redo));
+	ASSERT_EQ(std::string(buf->Rows()[0]), std::string("ab"));
+
+	// After selection, "b" should be the default redo from the branch point.
+	ASSERT_TRUE(Execute(ed, CommandId::Undo));
+	ASSERT_EQ(std::string(buf->Rows()[0]), std::string("a"));
+	ASSERT_TRUE(Execute(ed, CommandId::Redo));
+	ASSERT_EQ(std::string(buf->Rows()[0]), std::string("ab"));
+
+	validate_undo_tree(*u);
+}

tests/test_visual_line_mode.cc

@@ -0,0 +1,332 @@
+#include "Test.h"
+
+#include "Buffer.h"
+#include "Command.h"
+#include "Editor.h"
+
+#include <string>
+
+
+static std::string
+dump_buf(const Buffer &buf)
+{
+	std::string out;
+	for (const auto &r: buf.Rows()) {
+		out += static_cast<std::string>(r);
+		out.push_back('\n');
+	}
+	return out;
+}
+
+
+static std::string
+dump_bytes(const std::string &s)
+{
+	static const char *hex = "0123456789abcdef";
+	std::string out;
+	for (unsigned char c: s) {
+		out.push_back(hex[(c >> 4) & 0xF]);
+		out.push_back(hex[c & 0xF]);
+		out.push_back(' ');
+	}
+	return out;
+}
+
+
+TEST (VisualLineMode_BroadcastInsert)
+{
+	InstallDefaultCommands();
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+
+	Buffer b;
+	b.insert_text(0, 0, "foo\nfoo\nfoo\n");
+	b.SetCursor(1, 0); // fo|o
+	ed.AddBuffer(std::move(b));
+
+	ASSERT_TRUE(ed.CurrentBuffer() != nullptr);
+
+	// Enter visual-line mode and extend selection to 3 lines
+	ASSERT_TRUE(Execute(ed, std::string("visual-line-toggle")));
+	ASSERT_TRUE(Execute(ed, std::string("down"), std::string(), 2));
+
+	// Broadcast insert to all selected lines
+	ASSERT_TRUE(Execute(ed, std::string("insert"), std::string("X")));
+
+	const std::string got = dump_buf(*ed.CurrentBuffer());
+	// Note: buffers that end with a trailing '\n' have an extra empty row.
+	const std::string exp = "fXoo\nfXoo\nfXoo\n\n";
+	if (got != exp) {
+		std::cerr << "Expected (len=" << exp.size() << ") bytes: " << dump_bytes(exp) << "\n";
+		std::cerr << "Got      (len=" << got.size() << ") bytes: " << dump_bytes(got) << "\n";
+	}
+	ASSERT_TRUE(got == exp);
+}
+
+
+TEST (VisualLineMode_BroadcastInsert_UndoRedo)
+{
+	InstallDefaultCommands();
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+
+	Buffer b;
+	b.insert_text(0, 0, "foo\nfoo\nfoo\n");
+	b.SetCursor(1, 0); // fo|o
+	ed.AddBuffer(std::move(b));
+
+	ASSERT_TRUE(ed.CurrentBuffer() != nullptr);
+
+	ASSERT_TRUE(Execute(ed, std::string("visual-line-toggle")));
+	ASSERT_TRUE(Execute(ed, std::string("down"), std::string(), 2));
+
+	// Broadcast insert to all selected lines.
+	ASSERT_TRUE(Execute(ed, std::string("insert"), std::string("X")));
+	{
+		const std::string got = dump_buf(*ed.CurrentBuffer());
+		const std::string exp = "fXoo\nfXoo\nfXoo\n\n";
+		ASSERT_TRUE(got == exp);
+	}
+
+	// Undo should restore all affected lines in a single step.
+	ASSERT_TRUE(Execute(ed, std::string("undo")));
+	{
+		const std::string got = dump_buf(*ed.CurrentBuffer());
+		const std::string exp = "foo\nfoo\nfoo\n\n";
+		ASSERT_TRUE(got == exp);
+	}
+
+	// Redo should re-apply the whole insert.
+	ASSERT_TRUE(Execute(ed, std::string("redo")));
+	{
+		const std::string got = dump_buf(*ed.CurrentBuffer());
+		const std::string exp = "fXoo\nfXoo\nfXoo\n\n";
+		ASSERT_TRUE(got == exp);
+	}
+}
+
+
+TEST (VisualLineMode_BroadcastBackspace)
+{
+	InstallDefaultCommands();
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+
+	Buffer b;
+	b.insert_text(0, 0, "abcd\nabcd\nabcd\n");
+	b.SetCursor(2, 0); // ab|cd
+	ed.AddBuffer(std::move(b));
+
+	ASSERT_TRUE(Execute(ed, std::string("visual-line-toggle")));
+	ASSERT_TRUE(Execute(ed, std::string("down"), std::string(), 2));
+
+	ASSERT_TRUE(Execute(ed, std::string("backspace")));
+	const std::string got = dump_buf(*ed.CurrentBuffer());
+	// Note: buffers that end with a trailing '\n' have an extra empty row.
+	const std::string exp = "acd\nacd\nacd\n\n";
+	if (got != exp) {
+		std::cerr << "Expected (len=" << exp.size() << ") bytes: " << dump_bytes(exp) << "\n";
+		std::cerr << "Got      (len=" << got.size() << ") bytes: " << dump_bytes(got) << "\n";
+	}
+	ASSERT_TRUE(got == exp);
+}
+
+
+TEST (VisualLineMode_BroadcastBackspace_UndoRedo)
+{
+	InstallDefaultCommands();
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+
+	Buffer b;
+	b.insert_text(0, 0, "abcd\nabcd\nabcd\n");
+	b.SetCursor(2, 0); // ab|cd
+	ed.AddBuffer(std::move(b));
+
+	ASSERT_TRUE(Execute(ed, std::string("visual-line-toggle")));
+	ASSERT_TRUE(Execute(ed, std::string("down"), std::string(), 2));
+
+	ASSERT_TRUE(Execute(ed, std::string("backspace")));
+	{
+		const std::string got = dump_buf(*ed.CurrentBuffer());
+		const std::string exp = "acd\nacd\nacd\n\n";
+		ASSERT_TRUE(got == exp);
+	}
+
+	// Undo should restore all affected lines.
+	ASSERT_TRUE(Execute(ed, std::string("undo")));
+	{
+		const std::string got = dump_buf(*ed.CurrentBuffer());
+		const std::string exp = "abcd\nabcd\nabcd\n\n";
+		ASSERT_TRUE(got == exp);
+	}
+
+	// Redo should re-apply.
+	ASSERT_TRUE(Execute(ed, std::string("redo")));
+	{
+		const std::string got = dump_buf(*ed.CurrentBuffer());
+		const std::string exp = "acd\nacd\nacd\n\n";
+		ASSERT_TRUE(got == exp);
+	}
+}
+
+
+TEST (VisualLineMode_CancelWithCtrlG)
+{
+	InstallDefaultCommands();
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+
+	Buffer b;
+	b.insert_text(0, 0, "foo\nfoo\nfoo\n");
+	b.SetCursor(1, 0);
+	ed.AddBuffer(std::move(b));
+
+	ASSERT_TRUE(Execute(ed, std::string("visual-line-toggle")));
+	ASSERT_TRUE(Execute(ed, std::string("down"), std::string(), 2));
+
+	// C-g is mapped to "refresh" and should cancel visual-line mode.
+	ASSERT_TRUE(Execute(ed, std::string("refresh")));
+	ASSERT_TRUE(ed.CurrentBuffer() != nullptr);
+	ASSERT_TRUE(!ed.CurrentBuffer()->VisualLineActive());
+
+	// After cancel, edits should only affect the primary cursor line.
+	ASSERT_TRUE(Execute(ed, std::string("insert"), std::string("X")));
+	const std::string got = dump_buf(*ed.CurrentBuffer());
+	// Cursor is still on the last line we moved to (down, down).
+	const std::string exp = "foo\nfoo\nfXoo\n\n";
+	if (got != exp) {
+		std::cerr << "Expected (len=" << exp.size() << ") bytes: " << dump_bytes(exp) << "\n";
+		std::cerr << "Got      (len=" << got.size() << ") bytes: " << dump_bytes(got) << "\n";
+	}
+	ASSERT_TRUE(got == exp);
+}
+
+
+TEST (Yank_ClearsMarkAndVisualLine)
+{
+	InstallDefaultCommands();
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+
+	Buffer b;
+	b.insert_text(0, 0, "foo\nbar\n");
+	b.SetCursor(1, 0);
+	ed.AddBuffer(std::move(b));
+
+	ASSERT_TRUE(ed.CurrentBuffer() != nullptr);
+	Buffer *buf = ed.CurrentBuffer();
+
+	// Seed mark + visual-line highlighting.
+	buf->SetMark(buf->Curx(), buf->Cury());
+	ASSERT_TRUE(buf->MarkSet());
+
+	ASSERT_TRUE(Execute(ed, std::string("visual-line-toggle")));
+	ASSERT_TRUE(Execute(ed, std::string("down"), std::string(), 1));
+	ASSERT_TRUE(buf->VisualLineActive());
+
+	// Yank should clear mark and any highlighting.
+	ed.KillRingClear();
+	ed.KillRingPush("X");
+	ASSERT_TRUE(Execute(ed, std::string("yank")));
+
+	ASSERT_TRUE(!buf->MarkSet());
+	ASSERT_TRUE(!buf->VisualLineActive());
+}
+
+
+TEST (VisualLineMode_Yank_BroadcastsToBOL_AndUndo)
+{
+	InstallDefaultCommands();
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+
+	Buffer b;
+	b.insert_text(0, 0, "aa\nbb\ncc\n");
+	b.SetCursor(1, 0); // a|a
+	ed.AddBuffer(std::move(b));
+
+	ASSERT_TRUE(ed.CurrentBuffer() != nullptr);
+
+	// Enter visual-line mode and extend selection to 3 lines.
+	ASSERT_TRUE(Execute(ed, std::string("visual-line-toggle")));
+	ASSERT_TRUE(Execute(ed, std::string("down"), std::string(), 2));
+	ASSERT_TRUE(ed.CurrentBuffer()->VisualLineActive());
+
+	ed.KillRingClear();
+	ed.KillRingPush("X");
+
+	// Yank in visual-line mode should paste at BOL on every affected line.
+	ASSERT_TRUE(Execute(ed, std::string("yank")));
+	{
+		const std::string got = dump_buf(*ed.CurrentBuffer());
+		// Note: buffers that end with a trailing '\n' have an extra empty row.
+		const std::string exp = "Xaa\nXbb\nXcc\n\n";
+		if (got != exp) {
+			std::cerr << "Expected (len=" << exp.size() << ") bytes: " << dump_bytes(exp) << "\n";
+			std::cerr << "Got      (len=" << got.size() << ") bytes: " << dump_bytes(got) << "\n";
+		}
+		ASSERT_TRUE(got == exp);
+	}
+
+	// Undo should restore all affected lines in a single step.
+	ASSERT_TRUE(Execute(ed, std::string("undo")));
+	{
+		const std::string got = dump_buf(*ed.CurrentBuffer());
+		const std::string exp = "aa\nbb\ncc\n\n";
+		if (got != exp) {
+			std::cerr << "Expected (len=" << exp.size() << ") bytes: " << dump_bytes(exp) << "\n";
+			std::cerr << "Got      (len=" << got.size() << ") bytes: " << dump_bytes(got) << "\n";
+		}
+		ASSERT_TRUE(got == exp);
+	}
+
+	// Redo should re-apply the whole yank.
+	ASSERT_TRUE(Execute(ed, std::string("redo")));
+	{
+		const std::string got = dump_buf(*ed.CurrentBuffer());
+		const std::string exp = "Xaa\nXbb\nXcc\n\n";
+		ASSERT_TRUE(got == exp);
+	}
+}
+
+
+TEST (VisualLineMode_Highlight_IsPerLineCursorSpot)
+{
+	Buffer b;
+	// Note: buffers that end with a trailing '\n' have an extra empty row.
+	b.insert_text(0, 0, "abcd\nx\nhi\n");
+	// Place primary cursor on line 0 at column 3 (abc|d).
+	b.SetCursor(3, 0);
+
+	// Select lines 0..2 in visual-line mode.
+	b.VisualLineStart();
+	b.VisualLineSetActiveY(2);
+	ASSERT_TRUE(b.VisualLineActive());
+	ASSERT_TRUE(b.VisualLineStartY() == 0);
+	ASSERT_TRUE(b.VisualLineEndY() == 2);
+
+	// Line 0: "abcd" (len=4) => spot is 3
+	ASSERT_TRUE(b.VisualLineSpotSelected(0, 3));
+	ASSERT_TRUE(!b.VisualLineSpotSelected(0, 0));
+	ASSERT_TRUE(!b.VisualLineSpotSelected(0, 2));
+	ASSERT_TRUE(!b.VisualLineSpotSelected(0, 4));
+
+	// Line 1: "x" (len=1) => spot clamps to EOL (1)
+	ASSERT_TRUE(b.VisualLineSpotSelected(1, 1));
+	ASSERT_TRUE(!b.VisualLineSpotSelected(1, 0));
+
+	// Line 2: "hi" (len=2) => spot clamps to EOL (2)
+	ASSERT_TRUE(b.VisualLineSpotSelected(2, 2));
+	ASSERT_TRUE(!b.VisualLineSpotSelected(2, 0));
+
+	// Outside the selected line range should never be highlighted.
+	ASSERT_TRUE(!b.VisualLineSpotSelected(3, 0));
+}