DiffViewer: split MainViewModel and DiffPaneViewModel into focused coordinators

[geevensingh]

Problem

DiffViewer/ViewModels/MainViewModel.cs is 621 lines and DiffViewer/ViewModels/DiffPaneViewModel.cs is 690 lines. Both have grown to do too many unrelated things, which makes them harder to navigate, test, and refactor safely.

MainViewModel (621 lines) currently owns:

Concern	Lines	Notes
File-list context-menu commands	88-172	ShowInExplorer, OpenWithDefaultApp, OpenInExternalEditor, StageFile, DeleteFile, AddToGitignore, Copy* (5 variants), RefreshList
Diff-pane context-menu commands	175-227	StageHunkAtCaret, UnstageHunkAtCaret, RevertHunkAtCaret, OpenInExternalEditorAtLine
F7/F8 cross-file navigation	282-398	NavigateNextChange, NavigatePreviousChange, NavigateNextFile, NavigatePreviousFile, NavigateNextSection, NavigatePreviousSection, StepFile, StepSection, FindNextWithChanges, IndexOfSectionContaining
Toolbar toggle aliases	260-274	ToggleIgnoreWhitespace, ToggleVisibleWhitespace, etc.
Clipboard helper	421-439	CopyToClipboard with optional override
Repository-watcher coordination + write-op suspend tokens	30-39, 471-481, 600-684	_suspendTokens, _writeSnapshots, OnBeforeWriteOperation, OnAfterWriteOperation, RepoFileStatsSnapshot
Live-updates pause-handling	42-47, 540+	_missedChangeKindWhilePaused + OnDiffPanePropertyChanged
Settings hook for the View	65-69	SettingsService exposure

DiffPaneViewModel (690 lines) currently owns:

Concern	Lines	Notes
Settings ↔ observable-property bridging	137-217, 250-279, 530-561	Constructor seed, OnSettingsChanged, OnFontSizeChanged, Zoom*, PersistToolbarToSettings, OnXxxChanged (×5) — repetitive _suppressSettingsWrite plumbing
Color scheme management	175-217	CurrentColorScheme, ColorSchemeChanged, OnSettingsChanged color path
Diff load orchestration	295-360	LoadAsync
Auto-scroll variant	362-381	LoadAndScrollToFirstHunkAsync (just shipped)
Hunk navigation	599-669	TryNavigateNextHunkInFile, TryNavigatePreviousHunkInFile, IsAtLastHunk, IsAtFirstHunk, JumpToFirstHunk, JumpToLastHunk, JumpToHunk, RaiseHunkNav
Hunk patch building	(in HunkActionContext path)	HunkAtLine, BuildHunkPatchInputs, CurrentEntry
Viewport projection / scroll-by-fraction	127-135, 671-690+	Viewport, RequestScrollByFraction
Inline-document model	39, 45-56, 100, 503-504	InlineDocument, InlineLineHighlights, InlineLineToSourceLines
Placeholder logic	86-94, 312-321, 442-456	PlaceholderMessage, ResolvePlaceholderForShape, ResolveLargeFilePlaceholder

Why it matters

• New contributors have to load 1300+ lines of cross-cutting state to make any change.
• Tests are forced into one mega-fixture (MainViewModelKeyboardShortcutTests, MainViewModelContextMenuTests, DiffPaneViewModelTests).
• The auto-scroll race we just fixed (PR c4ed6f0) was hard to reason about because so much else lives in the same VM.

Suggested split

From MainViewModel:

1. HunkNavigationCoordinator — owns NavigateNext/PreviousChange/File/Section, StepFile, StepSection, FindNextWithChanges, IndexOfSectionContaining. Takes FileListViewModel, DiffPaneViewModel, and emits commands.
2. FileContextMenuCoordinator — owns ShowInExplorer, OpenWith*, StageFile, DeleteFile, AddToGitignore, Copy*, RefreshList. Takes IExternalAppLauncher, IGitWriteService, ISettingsService, ToastHandler.
3. HunkActionCoordinator — owns StageHunkAtCaret, UnstageHunkAtCaret, RevertHunkAtCaret, OpenInExternalEditorAtLine, plus TryGetHunkAction.
4. WriteOpWatcherSuspender — owns _suspendTokens, _writeSnapshots, OnBeforeWriteOperation, OnAfterWriteOperation, RepoFileStatsSnapshot. Pure plumbing; testable in isolation.

From DiffPaneViewModel:

5. ToolbarSettingsBridge — owns the seed-from-settings-on-construct dance, the _suppressSettingsWrite flag, the partial OnXxxChanged handlers, PersistToolbarToSettings, and the editor-appearance bridges (FontSize/FontFamily/TabWidth/etc.). About 80 lines lifted out, replaced with a single field ToolbarSettingsBridge _settings.

These are all mechanical extractions — the existing tests should continue to pass with no behaviour change.

Citations

• DiffViewer/ViewModels/MainViewModel.cs (621 lines)
• DiffViewer/ViewModels/DiffPaneViewModel.cs (690 lines)
• DiffViewer.Tests/ViewModels/MainViewModelKeyboardShortcutTests.cs, MainViewModelContextMenuTests.cs, DiffPaneViewModelTests.cs — the test files would also split naturally along these lines.

Severity

Low (no functional impact). Quality-of-life / maintainability win that compounds as more features land.