Fix writing mode: prevent per-frame override and disable syntax highlighting

apply_syntax_to_buffer() was called every frame and unconditionally reset
edit mode from the file extension, making it impossible to toggle out of
writing mode for .txt/.md files. Add edit_mode_detected_ flag to Buffer so
auto-detection runs once per buffer. Writing mode now also disables syntax
highlighting as intended. Propagate edit_mode_ through Buffer copy/move ops.

Co-Authored-By: Claude Opus 4.6 (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)

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()) {