Optimize ImGui renderer: viewport culling and width caching

Large files made idle CPU spike because every frame re-measured every
line, recompiled the search regex per line, and ran full per-line
rendering work even for off-screen rows.

- Render only visible rows (with a small margin for smooth scrolling)
  and advance the layout cursor to preserve total content height.
- Hoist the search std::regex compilation out of the per-line loop.
- Cache the max line width on the renderer; reset only when the buffer
  or font changes, not on every edit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Buffer.cc

@@ -231,10 +231,12 @@ Buffer::Buffer(const Buffer &other)
 	mark_set_         = other.mark_set_;
 	mark_curx_        = other.mark_curx_;
 	mark_cury_        = other.mark_cury_;
-	// Copy syntax/highlighting flags
-	version_        = other.version_;
-	syntax_enabled_ = other.syntax_enabled_;
-	filetype_       = other.filetype_;
+	// Copy edit mode + syntax/highlighting flags
+	edit_mode_          = other.edit_mode_;
+	edit_mode_detected_ = other.edit_mode_detected_;
+	version_            = other.version_;
+	syntax_enabled_     = other.syntax_enabled_;
+	filetype_           = other.filetype_;
 	// Fresh undo system for the copy
 	undo_tree_ = std::make_unique<UndoTree>();
 	undo_sys_  = std::make_unique<UndoSystem>(*this, *undo_tree_);
@@ -279,11 +281,13 @@ Buffer::operator=(const Buffer &other)
 	dirty_            = other.dirty_;
 	read_only_        = other.read_only_;
 	mark_set_         = other.mark_set_;
-	mark_curx_        = other.mark_curx_;
-	mark_cury_        = other.mark_cury_;
-	version_          = other.version_;
-	syntax_enabled_   = other.syntax_enabled_;
-	filetype_         = other.filetype_;
+	mark_curx_          = other.mark_curx_;
+	mark_cury_          = other.mark_cury_;
+	edit_mode_          = other.edit_mode_;
+	edit_mode_detected_ = other.edit_mode_detected_;
+	version_            = other.version_;
+	syntax_enabled_     = other.syntax_enabled_;
+	filetype_           = other.filetype_;
 	// Recreate undo system for this instance
 	undo_tree_ = std::make_unique<UndoTree>();
 	undo_sys_  = std::make_unique<UndoSystem>(*this, *undo_tree_);
@@ -326,13 +330,15 @@ Buffer::Buffer(Buffer &&other) noexcept
 	  undo_tree_(std::move(other.undo_tree_)),
 	  undo_sys_(std::move(other.undo_sys_))
 {
-	// Move syntax/highlighting state
-	version_          = other.version_;
-	syntax_enabled_   = other.syntax_enabled_;
-	filetype_         = std::move(other.filetype_);
-	highlighter_      = std::move(other.highlighter_);
-	content_          = std::move(other.content_);
-	rows_cache_dirty_ = other.rows_cache_dirty_;
+	// Move edit mode + syntax/highlighting state
+	edit_mode_          = other.edit_mode_;
+	edit_mode_detected_ = other.edit_mode_detected_;
+	version_            = other.version_;
+	syntax_enabled_     = other.syntax_enabled_;
+	filetype_           = std::move(other.filetype_);
+	highlighter_        = std::move(other.highlighter_);
+	content_            = std::move(other.content_);
+	rows_cache_dirty_   = other.rows_cache_dirty_;
 	// Update UndoSystem's buffer reference to point to this object
 	if (undo_sys_) {
 		undo_sys_->UpdateBufferReference(*this);
@@ -364,13 +370,15 @@ Buffer::operator=(Buffer &&other) noexcept
 	undo_tree_      = std::move(other.undo_tree_);
 	undo_sys_       = std::move(other.undo_sys_);
 
-	// Move syntax/highlighting state
-	version_          = other.version_;
-	syntax_enabled_   = other.syntax_enabled_;
-	filetype_         = std::move(other.filetype_);
-	highlighter_      = std::move(other.highlighter_);
-	content_          = std::move(other.content_);
-	rows_cache_dirty_ = other.rows_cache_dirty_;
+	// Move edit mode + syntax/highlighting state
+	edit_mode_          = other.edit_mode_;
+	edit_mode_detected_ = other.edit_mode_detected_;
+	version_            = other.version_;
+	syntax_enabled_     = other.syntax_enabled_;
+	filetype_           = std::move(other.filetype_);
+	highlighter_        = std::move(other.highlighter_);
+	content_            = std::move(other.content_);
+	rows_cache_dirty_   = other.rows_cache_dirty_;
 	// Update UndoSystem's buffer reference to point to this object
 	if (undo_sys_) {
 		undo_sys_->UpdateBufferReference(*this);

Buffer.h

@@ -517,6 +517,7 @@ public:
 	void SetEditMode(EditMode m)
 	{
 		edit_mode_ = m;
+		edit_mode_detected_ = true;
 	}
 
 
@@ -525,6 +526,13 @@ public:
 		edit_mode_ = (edit_mode_ == EditMode::Code)
 			             ? EditMode::Writing
 			             : EditMode::Code;
+		edit_mode_detected_ = true;
+	}
+
+
+	[[nodiscard]] bool EditModeDetected() const
+	{
+		return edit_mode_detected_;
 	}
 
 
@@ -660,6 +668,7 @@ private:
 
 	// Edit mode (code vs writing)
 	EditMode edit_mode_ = EditMode::Code;
+	bool edit_mode_detected_ = false; // true after initial auto-detection
 
 	// Syntax/highlighting state
 	std::uint64_t version_ = 0; // increment on edits

CLAUDE.md

@@ -0,0 +1,115 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+**kte** (Kyle's Text Editor) is a C++20 text editor with a terminal-first design (ncurses) and optional GUI frontends (ImGui via SDL2/OpenGL/Freetype, or Qt6). It uses a WordStar/VDE-style command model. The terminal editor is `kte`; the GUI editor is `kge`.
+
+## Build Commands
+
+```bash
+# Configure (from project root, build dir is "build")
+cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug -DBUILD_GUI=ON -DBUILD_TESTS=ON
+
+# Build everything
+cmake --build build
+
+# Build specific targets
+cmake --build build --target kte        # terminal editor
+cmake --build build --target kge        # GUI editor (requires -DBUILD_GUI=ON)
+cmake --build build --target kte_tests  # test suite
+
+# Run all tests
+cmake --build build --target kte_tests && ./build/kte_tests
+```
+
+There is no single-test runner; the test binary runs all tests. Tests use a minimal custom framework in `tests/Test.h` with `TEST()`, `ASSERT_EQ()`, `ASSERT_TRUE()`, `EXPECT_TRUE()` macros.
+
+### Key CMake Options
+
+| Flag | Default | Purpose |
+|------|---------|---------|
+| `BUILD_GUI` | ON | Build `kge` (ImGui GUI) |
+| `KTE_USE_QT` | OFF | Use Qt6 instead of ImGui for GUI |
+| `BUILD_TESTS` | ON | Build test suite |
+| `ENABLE_ASAN` | OFF | AddressSanitizer |
+| `KTE_STATIC_LINK` | OFF | Static linking (Linux only) |
+| `KTE_ENABLE_TREESITTER` | OFF | Tree-sitter syntax highlighting |
+
+### Nix
+
+`flake.nix` provides devshells: `default` (ImGui+debug tools), `terminal`, `qt`.
+
+### Docker (cross-platform Linux testing)
+
+```bash
+docker build -t kte-linux . && docker run --rm -v "$(pwd):/kte" kte-linux
+```
+
+## Architecture
+
+Three-layer design with strict frontend independence:
+
+```
+Frontend Layer (Terminal / ImGui / Qt / Test)
+  InputHandler.h, Renderer.h, Frontend.h interfaces
+        ↓
+Command Layer
+  CommandId enum → CommandRegistry → handler functions in Command.cc
+        ↓
+Core Model Layer
+  Editor → Buffer → PieceTable
+  UndoSystem (tree-based, records at PieceTable level)
+  SwapManager (crash recovery journal per buffer)
+```
+
+### Core Components
+
+- **PieceTable** (`PieceTable.h/.cc`) - Text storage. Lazy materialization; most ops work on the piece list directly. Line index and materialization caches must be invalidated on content changes.
+- **Buffer** (`Buffer.h/.cc`) - Wraps PieceTable. Prefer `GetLineView(row)` (zero-copy) or `GetLineString(row)` over `Rows()` (legacy, materializes all lines). All text mutations must go through PieceTable API (`insert_text`, `delete_text`) to ensure undo and swap recording work.
+- **Editor** (`Editor.h/.cc`) - Top-level state container. Primarily getters/setters; editing logic lives in commands.
+- **Command** (`Command.h/.cc`) - 120+ editing commands. This is the main place to add new editing operations. Register via `CommandRegistry::Register()` in `InstallDefaultCommands()`.
+- **UndoSystem/UndoTree/UndoNode** - Tree-based undo with branching. Group related ops with `buf.Undo()->BeginGroup()` / `EndGroup()`.
+- **Swap** (`Swap.h/.cc`) - Append-only crash recovery journal. Uses circuit breaker pattern for resilience. Files in `~/.local/state/kte/`.
+- **Syntax highlighting** (`syntax/`) - Pluggable per-language highlighters registered in `HighlighterRegistry`. Per-line caching with buffer version tracking.
+
+### Frontend Implementations
+
+Each frontend implements three interfaces (`Frontend.h`, `InputHandler.h`, `Renderer.h`):
+- **Terminal**: ncurses-based (always built)
+- **ImGui**: SDL2+OpenGL+Freetype (built with `-DBUILD_GUI=ON`)
+- **Qt**: Qt6 (built with `-DBUILD_GUI=ON -DKTE_USE_QT=ON`)
+- **Test**: Programmatic frontend for testing (always built, no UI deps)
+
+## Code Style
+
+- **C++20**, compiled with `-Wall -Wextra -Werror -pedantic`
+- **Clang** uses `-stdlib=libc++`
+- **Naming**: PascalCase for classes/methods, snake_case for variables, trailing underscore for private members (e.g., `pieces_`)
+- **Indentation**: Tabs
+- **Error handling**: Fallible ops use `bool func(args..., std::string &err)` pattern. Always clear `err` at start, capture `errno` immediately after syscall failure. Use EINTR-safe wrappers from `SyscallWrappers.h` instead of raw syscalls.
+- **ErrorHandler**: Centralized logging to `~/.local/state/kte/error.log` with severity levels (Info/Warning/Error/Critical).
+
+## Testing
+
+Tests live in `tests/test_*.cc`. Use `TestFrontend`/`TestInputHandler`/`TestRenderer` for integration tests that exercise the full Editor+Buffer+Command stack without UI dependencies.
+
+Key test files by area:
+- PieceTable: `test_piece_table.cc`
+- Buffer I/O: `test_buffer_io.cc`
+- Commands: `test_command_semantics.cc`
+- Search/replace: `test_search.cc`, `test_search_replace_flow.cc`
+- Undo: `test_undo.cc`
+- Swap (crash recovery): `test_swap_*.cc` (7 files)
+- Reflow: `test_reflow_paragraph.cc`, `test_reflow_indented_bullets.cc`
+- Integration: `test_daily_workflows.cc`
+
+## Important Caveats
+
+- `Buffer::Rows()` is legacy; use `GetLineView()` / `GetLineString()` in new code
+- `GetLineView()` returns a `string_view` valid only until next buffer modification
+- After editing ops, call `ensure_cursor_visible()` to update viewport
+- All source files are in the project root (no `src/` directory); tests are in `tests/`; syntax highlighters in `syntax/`; themes in `themes/`; embedded fonts in `fonts/`
+- External deps: `ext/imgui/` (Dear ImGui), `ext/tomlplusplus/` (TOML parser)
+- GUI config: `~/.config/kte/kge.toml` (TOML preferred over legacy INI)

CMakeLists.txt

@@ -4,7 +4,7 @@ project(kte)
 include(GNUInstallDirs)
 
 set(CMAKE_CXX_STANDARD 20)
-set(KTE_VERSION "1.11.0")
+set(KTE_VERSION "1.11.2")
 
 # Default to terminal-only build to avoid SDL/OpenGL dependency by default.
 # Enable with -DBUILD_GUI=ON when SDL2/OpenGL/Freetype are available.

Command.cc

@@ -1363,6 +1363,9 @@ cmd_toggle_edit_mode(const CommandContext &ctx)
 		b->ToggleEditMode();
 	}
 
+	// Writing mode disables syntax highlighting; code mode re-enables it.
+	b->SetSyntaxEnabled(b->GetEditMode() == EditMode::Code);
+
 	const char *mode_str = (b->GetEditMode() == EditMode::Writing) ? "writing" : "code";
 	ctx.editor.SetStatus(std::string("Mode: ") + mode_str);
 	return true;

ImGuiFrontend.cc

@@ -39,11 +39,13 @@ apply_syntax_to_buffer(Buffer *b, const GUIConfig &cfg)
 	if (!b)
 		return;
 
-	// Auto-detect edit mode from file extension
-	if (!b->Filename().empty())
+	// Auto-detect edit mode from file extension once per buffer so that
+	// manual toggles (C-k m / : mode) are not overridden every frame.
+	if (!b->EditModeDetected() && !b->Filename().empty())
 		b->SetEditMode(DetectEditMode(b->Filename()));
 
-	if (cfg.syntax) {
+	// Writing mode disables syntax; otherwise follow the global config.
+	if (cfg.syntax && b->GetEditMode() != EditMode::Writing) {
 		b->SetSyntaxEnabled(true);
 		b->EnsureHighlighter();
 		if (auto *eng = b->Highlighter()) {

ImGuiRenderer.cc

@@ -106,7 +106,6 @@ ImGuiRenderer::Draw(Editor &ed)
 		ImVec2 child_window_pos = ImGui::GetWindowPos();
 		float scroll_y          = ImGui::GetScrollY();
 		float scroll_x          = ImGui::GetScrollX();
-		std::size_t rowoffs     = 0; // we render from the first line; scrolling is handled by ImGui
 
 		// Synchronize buffer offsets from ImGui scroll if user scrolled manually
 		bool forced_scroll = false;
@@ -134,8 +133,65 @@ ImGuiRenderer::Draw(Editor &ed)
 		}
 		prev_buf_rowoffs_ = buf_rowoffs;
 		prev_buf_coloffs_ = buf_coloffs;
-		// Track the widest line in pixels for ImGui content width
-		float max_line_width_px = 0.0f;
+
+		// Max-line-width cache: reset when the buffer or font changes. We only
+		// measure visible lines per frame and maintain a running max, so the
+		// scrollbar may be slightly conservative on first view of a file until
+		// the user scrolls, but idle CPU drops dramatically on large files.
+		// Edits bump the buffer version but we intentionally do NOT reset the
+		// max on version changes — keeping a conservative (possibly too-wide)
+		// scrollbar is preferable to per-keystroke jitter.
+		{
+			ImFont *cur_font = ImGui::GetFont();
+			float cur_fsize  = ImGui::GetFontSize();
+			if (buf != max_width_buf_
+			    || cur_font != max_width_font_
+			    || cur_fsize != max_width_font_size_) {
+				max_width_buf_       = buf;
+				max_width_font_      = cur_font;
+				max_width_font_size_ = cur_fsize;
+				max_width_px_        = 0.0f;
+			}
+			max_width_version_ = buf->Version();
+		}
+
+		// Hoist the search-regex compilation out of the per-line loop. Compiling
+		// std::regex per line per frame was a large source of idle CPU on macOS.
+		const bool search_mode = ed.SearchActive() && !ed.SearchQuery().empty();
+		const bool regex_mode  = search_mode && ed.PromptActive() && (
+			ed.CurrentPromptKind() == Editor::PromptKind::RegexSearch ||
+			ed.CurrentPromptKind() == Editor::PromptKind::RegexReplaceFind);
+		std::regex search_rx;
+		bool search_rx_valid = false;
+		if (regex_mode) {
+			try {
+				search_rx       = std::regex(ed.SearchQuery());
+				search_rx_valid = true;
+			} catch (const std::regex_error &) {
+				search_rx_valid = false;
+			}
+		}
+
+		// Compute the visible row range and skip rendering work for off-screen
+		// lines. ImGui clips drawing, but string allocation, tab expansion,
+		// syntax-highlight lookups, and width measurement are not free.
+		const std::size_t total_rows = lines.size();
+		std::size_t first_vis = 0;
+		std::size_t last_vis  = total_rows; // exclusive
+		if (row_h > 0.0f && total_rows > 0) {
+			const long margin = 4; // render a few extra rows above/below for smoother scrolling
+			long fv           = static_cast<long>(std::floor(scroll_y / row_h)) - margin;
+			long lv           = static_cast<long>(
+				std::ceil((scroll_y + child_h_plan) / row_h)) + margin;
+			if (fv < 0) fv = 0;
+			if (lv < 0) lv = 0;
+			if (static_cast<std::size_t>(fv) > total_rows)
+				fv = static_cast<long>(total_rows);
+			if (static_cast<std::size_t>(lv) > total_rows)
+				lv = static_cast<long>(total_rows);
+			first_vis = static_cast<std::size_t>(fv);
+			last_vis  = static_cast<std::size_t>(lv);
+		}
 
 		// Mark selection state (mark -> cursor), in source coordinates
 		bool sel_active    = false;
@@ -257,7 +313,13 @@ ImGuiRenderer::Draw(Editor &ed)
 		if (mouse_selecting_ && ImGui::IsMouseReleased(ImGuiMouseButton_Left)) {
 			mouse_selecting_ = false;
 		}
-		for (std::size_t i = rowoffs; i < lines.size(); ++i) {
+		// Advance the cursor Y so the first visible line draws at its correct
+		// scroll position. Skipped rows simply leave the layout cursor untouched.
+		if (first_vis > 0) {
+			ImGui::SetCursorPosY(ImGui::GetCursorPosY() +
+			                     static_cast<float>(first_vis) * row_h);
+		}
+		for (std::size_t i = first_vis; i < last_vis; ++i) {
 			// Capture the screen position before drawing the line
 			ImVec2 line_pos  = ImGui::GetCursorScreenPos();
 			std::string line = static_cast<std::string>(lines[i]);
@@ -291,24 +353,22 @@ ImGuiRenderer::Draw(Editor &ed)
 			};
 
 			// Compute search highlight ranges for this line in source indices
-			bool search_mode = ed.SearchActive() && !ed.SearchQuery().empty();
 			std::vector<std::pair<std::size_t, std::size_t> > hl_src_ranges;
 			if (search_mode) {
-				// If we're in RegexSearch or RegexReplaceFind mode, compute ranges using regex; otherwise plain substring
-				if (ed.PromptActive() && (
-					    ed.CurrentPromptKind() == Editor::PromptKind::RegexSearch || ed.
-					    CurrentPromptKind() == Editor::PromptKind::RegexReplaceFind)) {
-					try {
-						std::regex rx(ed.SearchQuery());
-						for (auto it = std::sregex_iterator(line.begin(), line.end(), rx);
-						     it != std::sregex_iterator(); ++it) {
-							const auto &m  = *it;
-							std::size_t sx = static_cast<std::size_t>(m.position());
-							std::size_t ex = sx + static_cast<std::size_t>(m.length());
-							hl_src_ranges.emplace_back(sx, ex);
+				// In regex mode, reuse the compiled regex hoisted above the loop.
+				if (regex_mode) {
+					if (search_rx_valid) {
+						try {
+							for (auto it = std::sregex_iterator(line.begin(), line.end(), search_rx);
+							     it != std::sregex_iterator(); ++it) {
+								const auto &m  = *it;
+								std::size_t sx = static_cast<std::size_t>(m.position());
+								std::size_t ex = sx + static_cast<std::size_t>(m.length());
+								hl_src_ranges.emplace_back(sx, ex);
+							}
+						} catch (const std::regex_error &) {
+							// ignore invalid patterns here; status line already shows the error
 						}
-					} catch (const std::regex_error &) {
-						// ignore invalid patterns here; status line already shows the error
 					}
 				} else {
 					const std::string &q = ed.SearchQuery();
@@ -483,18 +543,29 @@ ImGuiRenderer::Draw(Editor &ed)
 				ImGui::GetWindowDrawList()->AddRectFilled(p0, p1, col);
 			}
 
-			// Track widest line for content width reporting
+			// Track widest line for content width reporting. We only measure
+			// visible lines and fold the result into a monotonic max cached on
+			// the renderer (see max_width_* members). Off-screen lines are not
+			// measured per frame; if the user scrolls or edits, the cache is
+			// refreshed accordingly.
 			if (!expanded.empty()) {
 				float line_w = ImGui::CalcTextSize(expanded.c_str()).x;
-				if (line_w > max_line_width_px)
-					max_line_width_px = line_w;
+				if (line_w > max_width_px_)
+					max_width_px_ = line_w;
 			}
 		}
-		// Report content width to ImGui so horizontal scrollbar works correctly.
-		if (max_line_width_px > 0.0f) {
-			ImGui::SetCursorPosX(max_line_width_px);
-			ImGui::Dummy(ImVec2(0, 0));
+		// After the visible-range loop, advance the layout cursor to the end of
+		// the (virtual) content so ImGui sees the correct total size. Vertical
+		// height comes from total_rows; horizontal width comes from the cached
+		// max line width. A Dummy at the final position records both.
+		if (total_rows > last_vis) {
+			ImGui::SetCursorPosY(ImGui::GetCursorPosY() +
+			                     static_cast<float>(total_rows - last_vis) * row_h);
 		}
+		if (max_width_px_ > 0.0f) {
+			ImGui::SetCursorPosX(max_width_px_);
+		}
+		ImGui::Dummy(ImVec2(0, 0));
 		// Synchronize cursor and scrolling after rendering all lines so content size is known.
 		{
 			float child_h_actual = ImGui::GetWindowHeight();

ImGuiRenderer.h

@@ -2,8 +2,13 @@
  * ImGuiRenderer - ImGui-based renderer for GUI mode
  */
 #pragma once
+#include <cstdint>
+#include <string>
 #include "Renderer.h"
 
+struct ImFont;
+class Buffer;
+
 class ImGuiRenderer final : public Renderer {
 public:
 	ImGuiRenderer() = default;
@@ -20,4 +25,13 @@ private:
 	float prev_scroll_y_   = -1.0f;
 	float prev_scroll_x_   = -1.0f;
 	bool mouse_selecting_  = false;
+
+	// Max-line-width cache for the horizontal scrollbar. Measuring every line
+	// every frame is prohibitively expensive on large files; we only update the
+	// running max from visible lines and reset when buffer/version/font changes.
+	const Buffer *max_width_buf_ = nullptr;
+	std::uint64_t max_width_version_ = 0;
+	ImFont *max_width_font_ = nullptr;
+	float max_width_font_size_ = 0.0f;
+	float max_width_px_ = 0.0f;
 };

main.cc

@@ -12,6 +12,7 @@
 #include <random>
 #include <thread>
 #include <signal.h>
+#include <filesystem>
 #include <string>
 #include <unistd.h>
 #include <sys/stat.h>
@@ -255,7 +256,18 @@ main(int argc, char *argv[])
 					// Fall through: not a +number, treat as filename starting with '+'
 				}
 
-				const std::string path = arg;
+				// Resolve to absolute path now, before any
+				// chdir (macOS GUI changes CWD to HOME before
+				// deferred opens are processed).
+				std::string path = arg;
+				try {
+					std::filesystem::path p(path);
+					if (p.is_relative()) {
+						path = std::filesystem::absolute(p).string();
+					}
+				} catch (...) {
+					// Fall through with original path
+				}
 				editor.RequestOpenFile(path, pending_line);
 				pending_line = 0; // consumed (if set)
 			}