refactor(diff): single DiffViewer replaces DiffModal + WorktreeModal + PlanningDiff

docs/superpowers/plans/2026-06-19-unify-diff-viewer.md

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+# Phase 5 — DiffViewer (A1 + B2)
+
+Date: 2026-06-23
+Umbrella: `docs/superpowers/plans/2026-06-19-feature-unification-plan.md`
+Design: `docs/superpowers/specs/2026-06-19-feature-unification-design.md` (A1, B2)
+
+## Goal
+
+One diff component replaces the three parallel read-only diff windows:
+`DiffModalViewModel`/View, `WorktreeModalViewModel`/View, `PlanningDiffViewModel`/View.
+**Merge editor (`ConflictResolverViewModel`) is untouched** — per the design's hard
+decision; the viewer only *opens* it on conflict via the existing Merge flow.
+
+All three are already master-detail: **left nav pane + right `DiffLinesView`**. They
+differ only in left-pane content, chrome, and data source — so they collapse into one
+shell with a source mode.
+
+## Decisions (Mika, 2026-06-23)
+
+- **File nav = file-tree** (folder-grouped), not a flat list. Port `WorktreeModal`'s tree
+  + the Avalonia-12 `TreeView.SelectionChanged` workaround. Carry per-file status + +adds/
+  −dels into the tree rows (from the parsed `DiffFileViewModel`).
+- Planning keeps its **subtask-list + combined-mode toggle**; the branch source keeps its
+  **Merge** button.
+
+## Target
+
+### Shared types → `ViewModels/Modals/DiffModels.cs` (new, same namespace)
+
+Move out of the to-be-deleted VMs so `UnifiedDiffParser`/`DiffLinesView` keep compiling:
+`DiffLineKind`, `DiffFileStatus`, `DiffLineViewModel`, `DiffFileViewModel` (from
+`DiffModalViewModel.cs`), `SubtaskDiffRow` (from `PlanningDiffViewModel.cs`). Add new
+`DiffTreeNodeViewModel` (dir/file node; file leaves hold their `DiffFileViewModel`).
+
+### `DiffViewerViewModel` (`ViewModels/Modals/DiffViewerViewModel.cs`, new)
+
+ctor `(GitService git, IWorkerClient worker)`. A `DiffViewerMode { Files, Planning }`.
+
+- **File sources** (replaces DiffModal + WorktreeModal): config props `WorktreePath`,
+  `BaseRef`, `HeadCommit`, `FromCommitRange`, `TaskId`, `TaskTitle` + `ShowMergeModal`/
+  `ResolveMergeVm` delegates. `LoadAsync` pulls the whole diff via GitService
+  (`GetCommitRangeDiffAsync` | `GetBranchDiffAsync` | `GetDiffAsync`), parses with
+  `UnifiedDiffParser.Parse`, builds `FileTree`. `SelectedNode` (leaf) → `SelectedFile`
+  (header + binary/empty placeholders + `Lines`). Commit-range null-guard → "no longer
+  available" (preserve DiffModal behavior). `MergeCommand` (CanMerge = TaskId +
+  delegates) opens the MergeModal, closes on merged/routed (verbatim from DiffModal).
+- **Planning source** (replaces PlanningDiff): config `PlanningTaskId`, `TargetBranch`.
+  `LoadAsync` pulls `GetPlanningAggregateAsync` → `Subtasks`; `SelectedSubtask` →
+  `DisplayedDiff`; `IsCombinedMode` toggle → `BuildPlanningIntegrationBranchAsync`
+  (success → combined diff; conflict → `CombinedWarning` with subtask + file count;
+  null → hub-error warning). `DisplayedDiff` → flattened `DiffLines` (right pane).
+- Shared: `StatusMessage`, `CloseAction`, `CloseCommand`.
+
+### `DiffViewerView` (`Views/Modals/DiffViewerView.axaml` + `.cs`, new)
+
+`ModalShell`-based window. Left pane: `TreeView` (Files mode) or subtask `ListBox`
+(Planning mode), toggled by mode. Right pane: the DiffModal file pane (header + binary/
+empty/no-changes placeholders + `DiffLinesView Lines="SelectedFile.Lines"`) in Files mode,
+or `DiffLinesView Lines="DiffLines"` in Planning mode. Toolbar: combined toggle + warning
++ loading (Planning). Footer: Merge button (Files mode, CanMerge). Code-behind: `CloseAction`,
+the `TreeView.SelectionChanged` → `SelectedNode` workaround, dir-row tap-to-expand.
+
+### Re-point the 3 doors → one viewer
+
+- **`MergeSectionViewModel`**: `OpenDiffAsync` builds a Files-mode `DiffViewerViewModel`
+  (+ ShowMergeModal/ResolveMergeVm) and calls a single `ShowDiffViewer` delegate;
+  `ReviewCombinedDiffAsync` builds a Planning-mode one and calls the *same* delegate.
+  Replaces `ShowDiffModal` + `ShowPlanningDiffModal` with one `Func<DiffViewerViewModel,Task>
+  ShowDiffViewer`; keeps `ShowMergeModal`. (Resolve the VM via `_services`.)
+- **`DetailsIslandView.axaml.cs`**: replace the two `ShowDiffModal`/`ShowPlanningDiffModal`
+  wirings (→ `DiffModalView`/`PlanningDiffView`) with one `ShowDiffViewer` (→ `DiffViewerView`).
+  Keep `ShowMergeModal`.
+- **`WorktreesOverviewModalViewModel`**: `ShowDiff` builds a Files-mode viewer (worktree path
+  + base). Change `_diffVmFactory` from `Func<WorktreeModalViewModel>` to
+  `Func<DiffViewerViewModel>`; `ShowDiffAction` stays `Action<DiffViewerViewModel>`.
+- **`WindowDialogService.cs`**: `ShowDiffAction` → `new DiffViewerView` + `LoadAsync` + show.
+- **`Program.cs`**: register `DiffViewerViewModel` (transient) + `Func<DiffViewerViewModel>`;
+  drop the `WorktreeModalViewModel` registration.
+
+### Delete
+
+`DiffModalViewModel.cs`, `WorktreeModalViewModel.cs`, `PlanningDiffViewModel.cs`,
+`DiffModalView.axaml(.cs)`, `WorktreeModalView.axaml(.cs)`, `PlanningDiffView.axaml(.cs)`.
+
+### Localization
+
+Reuse existing keys in the merged view (`modals.diff.*` for the file pane, `planning.diff.*`
+for the planning toolbar). Prune clearly-orphaned `modals.worktree.*` if trivial; keep en/de
+parity.
+
+## Tests
+
+Replace `DiffModalViewModelTests` + `PlanningDiffViewModelTests` with
+`DiffViewerViewModelTests` preserving the behaviors: commit-range null-guard → unavailable;
+planning init populates + selects first; subtask select → DisplayedDiff; combined toggle
+success/conflict/null. `WorktreesOverviewBatchMergeTests` compiles unchanged (`() => null!`
+satisfies the new Func type). `UnifiedDiffParserTests` unchanged.
+
+## Acceptance
+
+- `dotnet build -c Release` clean (App); `Ui.Tests` + `Localization.Tests` green.
+- One viewer reached from all 3 doors; old VMs/views deleted; merge editor untouched.
+- Visual gap flagged: Details "Open Diff" (dirty + post-merge commit-range), Worktrees-
+  Overview "Show Diff" (tree), Details "Review Combined Diff" (subtasks + combined toggle),
+  and the Merge button still opens the merge form / resolver on conflict.
+
+## Commit
+
+`refactor(diff): single DiffViewer replaces DiffModal + WorktreeModal + PlanningDiff`.
+Stage by path (exclude concurrent peers' files). Then Phase 3 (WorktreeActions) follows as
+its own slice, reusing this viewer.