# Diff Viewer: Side-by-Side, Syntax Highlighting, Word Diff Date: 2026-08-07 Status: approved (design), not implemented ## Problem The diff viewer renders every change as a flat unified stream. Reading what actually changed inside a modified line means mentally aligning a `−` row with a `+` row several rows below it. The user reads diffs faster side by side. Three gaps, all in the same surface: 1. No side-by-side mode. 2. No syntax highlighting — the merge editor (`ConflictResolverView`) already has it via TextMate, the diff viewer does not. 3. No intra-line (word) highlighting, so a one-character change looks like a whole-line rewrite. ## Current state | Concern | Where | |---|---| | Parsing | `src/ClaudeDo.Ui/ViewModels/Modals/UnifiedDiffParser.cs` → `DiffFileViewModel.Lines` | | Models | `src/ClaudeDo.Ui/ViewModels/Modals/DiffModels.cs` (`DiffLineViewModel{Kind,OldNo,NewNo,Text}`, `DiffLineKind{Add,Del,Ctx,File}`) | | Rendering | `src/ClaudeDo.Ui/Views/Controls/DiffLinesView.axaml` — non-virtualized `ItemsControl`, one `Border`+`Grid`+4 `TextBlock`s per line, `TextWrapping="NoWrap"` | | Host | `src/ClaudeDo.Ui/Views/Modals/DiffViewerView.axaml` — Files mode (line 154, `SelectedFile.Lines`) and Planning mode (line 162, flattened `DiffLines` across all files) | | Highlighting reference | `src/ClaudeDo.Ui/Views/Conflicts/ConflictResolverView.axaml.cs:80-83` — `RegistryOptions(ThemeName.DarkPlus)` + `InstallTextMate` + `SetGrammar` by file extension | `Avalonia.AvaloniaEdit`, `AvaloniaEdit.TextMate` and `TextMateSharp.Grammars` are already referenced in `ClaudeDo.Ui.csproj`. ## Decision Replace `DiffLinesView` with an AvaloniaEdit-based control. TextMate highlighting is bound to the `TextEditor` control; it cannot be lifted into `TextBlock` inlines without reimplementing the scope→brush layer that `AvaloniaEdit.TextMate` already provides. Building split/wrap/word diff on the `TextBlock` model first and swapping the renderer later would be throwaway work. Side effect worth having: AvaloniaEdit virtualizes, which removes the current non-virtualized `ItemsControl` as a scaling limit on large diffs. Rejected: keeping the `ItemsControl` and hand-rolling highlighting from `TextMateSharp` tokenization — same output, materially more code, and a second highlighting path to maintain alongside the merge editor's. ## Layout semantics Left pane is the **old** state, right pane is the **new** state — each side carries the complete version of the hunk, not "removals here, additions there". ``` LEFT (old) RIGHT (new) 12 public void Save() 12 public void Save() 13 var x = 1; 13 var x = 2; ← word diff on `1` / `2` 14 Log("old"); · (filler) · (filler) 14 Log("new"); 15 } 15 } ``` ## Components ### 1. `DiffAlignment` (new, pure) `src/ClaudeDo.Ui/ViewModels/Modals/DiffAlignment.cs` Turns `IReadOnlyList` into a render-ready `AlignedDiff`. No Avalonia types, fully unit-testable. ```csharp public enum AlignedSide { Ctx, Del, Add, Filler, Gap } public readonly record struct TextSpan(int Start, int Length); public sealed record SplitRow( AlignedSide LeftKind, int? OldNo, string LeftText, IReadOnlyList LeftSpans, AlignedSide RightKind, int? NewNo, string RightText, IReadOnlyList RightSpans); public sealed record UnifiedRow( AlignedSide Kind, int? OldNo, int? NewNo, string Text, IReadOnlyList Spans); public sealed record AlignedDiff( IReadOnlyList SplitRows, string LeftText, string RightText, IReadOnlyList UnifiedRows, string UnifiedText); ``` Row index `i` maps to document line `i + 1` in the corresponding text. That mapping is the contract the margin and both renderers depend on. **Pairing.** Walk the lines. A `Ctx` run emits rows with the same text on both sides. A change block (a `Del` run followed by an `Add` run) pairs index-wise up to `min(delCount, addCount)`; the overhang gets `Filler` rows on the opposite side. **Gaps.** The parser drops `@@` headers, so a skipped region shows up as a jump in `OldNo`/`NewNo` between consecutive lines. `DiffAlignment` detects that jump and inserts a `Gap` row on both sides. The parser is not touched. **Word diff.** Only for `(Del, Add)` rows that are paired 1:1. Tokenize each side into runs of word characters / whitespace / single punctuation, run an LCS over the tokens, and emit the changed token runs as character spans per side. Two guards, both `const` and both covered by tests: - Skip when either side exceeds `MaxWordDiffChars = 2000` — LCS cost, and such lines are unreadable as word diffs anyway. - Skip when token similarity is below `MinWordDiffSimilarity = 0.5` (common tokens / max token count). Below that the two lines are unrelated rewrites and per-word tinting is noise. ### 2. `DiffTextView` (new control, replaces `DiffLinesView`) `src/ClaudeDo.Ui/Views/Controls/DiffTextView.axaml` + `.axaml.cs` Styled properties: | Property | Type | Meaning | |---|---|---| | `File` | `DiffFileViewModel?` | Source; the control aligns it and caches the `AlignedDiff` per file instance | | `Mode` | `DiffViewMode` (`Unified`\|`Split`) | Layout | | `WrapLines` | `bool` | Bound to each editor's `WordWrap` | Two `TextEditor`s in a two-column grid, both `IsReadOnly=true`, `ShowLineNumbers=false`. `Unified` mode collapses the right editor and spans the left one across both columns, feeding it `UnifiedText`. `Split` mode shows both, fed `LeftText` / `RightText`. Per editor: - **TextMate**: one shared `RegistryOptions(ThemeName.DarkPlus)`; grammar resolved from `File.Path`'s extension via `GetLanguageByExtension` → `GetScopeByLanguageId` → `SetGrammar`, exactly as `ConflictResolverView.ApplyGrammar` does. No extension match → no grammar, plain text. - **`DiffLineNumberMargin : AbstractMargin`** — draws line numbers from the row list. Split: old numbers left, new numbers right. Unified: two number columns in one margin. `Filler` and `Gap` rows draw nothing. - **`DiffLineBackgroundRenderer : IBackgroundRenderer`** — full-width tint per visual line by row kind: add / del / filler / gap / ctx. - **`WordDiffRenderer : IBackgroundRenderer`** — stronger tint over the changed spans, via `BackgroundGeometryBuilder` at `lineStartOffset + span.Start`. Both renderers resolve rows through a single `Func` keyed by document line. **Highlighting on fragments.** Only hunks are in the document, not whole files, so TextMate's line-by-line state can be wrong at a fragment boundary (a line inside a block comment may be highlighted as code). Accepted — the same is true of every fragment-based diff viewer. **Colors.** Line tints stay the existing low-alpha `RunningTintBrush` / `ErrorTintBrush` so syntax foregrounds remain legible. The per-line foreground recolor from `DiffLinesView` (green/red text) is dropped — syntax colors take over. `Filler` gets a new dim token brush, `Gap` renders as a dim `⋯` separator row. **Scroll sync** (split only), modeled on `ConflictResolverView.HookScrollSync` — find each editor's descendant `ScrollViewer`, guard re-entry with a `_syncing` flag: - `WrapLines = false`: sync `Offset.Y` directly. Line heights match, so alignment is exact. - `WrapLines = true`: line heights diverge. Sync on the first visible document line instead (`ScrollToLine`), which keeps the top of the viewport aligned and lets rows drift downward. ### 3. Host changes `DiffViewerView.axaml`: - Header gains a segmented Unified/Split toggle and a wrap toggle. - Files mode: `DiffLinesView Lines="{Binding SelectedFile.Lines}"` → `DiffTextView File="{Binding SelectedFile}"`. - Planning mode: the flattened single-stream `DiffLines` view is replaced by an `ItemsControl` over the subtask's parsed files, each item a file header plus its own `DiffTextView`. One editor can only carry one grammar, so per-file editors are required for highlighting to work at all here. `DiffViewerViewModel` exposes the parsed per-file list for the selected subtask; `DiffLines` and `UnifiedDiffParser.Flatten` lose their last consumer and are removed. `DiffLinesView.axaml` + `.axaml.cs` are deleted once both usages are migrated. ### 4. Persistence `src/ClaudeDo.Ui/AppSettings.cs` (`~/.todo-app/ui.config.json`) already holds UI-only preferences (`Language`, `AccentPreset`) with plain `Load()`/`Save()`. Two properties are added there: ```csharp public string DiffViewMode { get; set; } = "unified"; // "unified" | "split" public bool DiffWrapLines { get; set; } ``` `DiffViewerViewModel` takes the injected `AppSettings`, seeds its toggles on open and calls `Save()` when either changes. No database column, no EF migration, no hub method — a view preference does not belong in `AppSettingsEntity`. ### 5. Localization New keys in both `locales/en.json` and `locales/de.json` (Localization.Tests enforces parity): `diff.view.unified`, `diff.view.split`, `diff.view.wrap`. ## Testing `tests/ClaudeDo.Ui.Tests` — `DiffAlignment` is pure and carries the logic worth testing: - Context-only diff → identical rows on both sides, no fillers. - Equal-size change block → 1:1 pairing, no fillers. - Unequal change block (3 del / 5 add) → 3 paired rows + 2 right-side rows with left fillers. - Add-only and delete-only blocks → fillers on the opposite side throughout. - Non-contiguous line numbers → exactly one `Gap` row inserted. - Word diff: single-token change yields one span per side at the right offsets. - Word diff skipped above `MaxWordDiffChars` and below `MinWordDiffSimilarity`. - Row index ↔ document line mapping holds for both `SplitRows` and `UnifiedRows`. - Binary file and empty-content file → empty `AlignedDiff`, no crash. `AppSettings` round-trip: persisted mode and wrap survive `Save()`/`Load()`. Rendering (margin, both renderers, scroll sync, TextMate colors) is not unit-testable here and is an explicit manual visual pass — see Open items. ## Known limitations 1. **Wrap + split drift.** With wrap on, the two panes align at the top of the viewport but rows drift apart further down. Per-line vertical alignment as VS Code does it is out of scope. 2. **Fragment highlighting.** See above — highlighting state can be wrong at hunk boundaries. 3. **Editors per file in Planning mode.** A subtask touching many files instantiates one editor per file. Same cost as viewing those files individually in Files mode; not capped. If it proves slow, the fix is lazy instantiation on expand, not a silent truncation. ## Open items (manual verification) - Visual pass on both modes: tints legible over DarkPlus syntax colors; line numbers aligned; filler and gap rows readable. - Scroll sync with wrap off (exact) and wrap on (top-anchored). - Planning mode with a multi-file subtask. - Toggle state survives an app restart. ## Docs to update on completion - `src/ClaudeDo.Ui/CLAUDE.md` — Views/Controls list and the "Diff & Conflicts" section still name `DiffLinesView`. - `docs/explore-notes/review-merge.md` — diff stack description + "verified against" commit.