# 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<DiffLineViewModel>` 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<TextSpan> LeftSpans,
    AlignedSide RightKind, int? NewNo, string RightText, IReadOnlyList<TextSpan> RightSpans);

public sealed record UnifiedRow(
    AlignedSide Kind, int? OldNo, int? NewNo, string Text, IReadOnlyList<TextSpan> Spans);

public sealed record AlignedDiff(
    IReadOnlyList<SplitRow> SplitRows,   string LeftText, string RightText,
    IReadOnlyList<UnifiedRow> 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<int, RowInfo?>` 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.
