Add worktree labels to the UI and worktree switching to the dialog

[geevensingh]

Why

DiffViewer already worked when pointed at a linked worktree, but nothing on screen acknowledged it and there was no way to get to one except typing its path from memory.

The visible symptom was in the recents dropdown. Row titles came from Path.GetFileName(repoPath), which for a worktree is usually the branch name and never mentions the repository. A worktree at ...\copilot-worktrees\DiffViewer\feature-x rendered as feature-x - HEAD -> WT, so two worktrees of one repo were effectively indistinguishable and the word "DiffViewer" appeared nowhere.

What changed

Legibility. Recent rows now render as DiffViewer [feature-x] - HEAD -> WT, and the window title appends the worktree name after the path. The labels are captured at launch time and persisted alongside the existing display-only fields, so the dropdown does no disk IO at render time. Rows written by earlier versions load with null labels, fall back to the old rendering, and heal on next launch.

Switching. The repo-path row in the New diff dialog gained a Worktree... button, and the context bar gained a matching Worktree button that re-opens the comparison you are already looking at rooted at a different checkout, with both sides unchanged. Both list the main worktree first with each entry's checked-out branch and path.

New seams. GitWorktreeLayout does libgit2-free path reasoning about git's on-disk worktree layout (commondir pointers, .git-as-a-file pointers, repository naming). IGitWorktreeEnumerator lists worktrees; WorktreePickerViewModel mirrors the existing RefPickerViewModel shape, and the context-bar switcher reuses it with a write-back that runs a context switch instead of editing a text box.

Things worth a careful look

Two libgit2 quirks drove the enumerator's design. Both were verified empirically against a real repo rather than assumed:

• libgit2's worktree list omits the main worktree entirely, so it is synthesized by resolving the commondir pointer.
• A pruned worktree (directory deleted out from under git) is still listed, but every property access throws NullReferenceException. Names therefore come from the <common>/worktrees/* administrative directories, which survive the deletion. Such worktrees are shown as unavailable rather than silently dropped.

The picker is offered on every local form, not just working-tree ones. I initially argued against this. Two worktrees share an object database and all shared refs, so main..feature resolves identically from either. But HEAD is per-worktree and the dialog advertises HEAD~3 as valid input, so the choice genuinely changes the answer for commit-ish forms too.

LocalRepoFormViewModelBase is an extraction, and it is the largest part of the diff. The five local New-diff forms each carried their own copy of the repo path, its canonicalization, and the deferred-error handling, three of them verbatim identical, and the worktree picker would have been a sixth copy. NewDiffDialog.xaml duplicated the repo-path Grid five times likewise and is now one RepoPathField control, which also retires the Browse handler's Tag="RepoPath" visual-tree walk in favour of a named element. The extraction is intended to be behavior-preserving; the full pre-existing suite passing unchanged is the main evidence for that, and the ordering of canonicalization vs Validate() vs side effects like TrySeedDefaultPartner is the part most worth a second pair of eyes.

The third commit fixes a latent trap found in review. RepositoryShape.CommonGitDirectory is documented as identical across every worktree of a repository, which is what makes "same repo, different checkout" recognizable. It was not: libgit2 reports a main worktree's git dir with a trailing separator while the commondir pointer resolves without one. Nothing consumed the field yet, so it was a trap for the first caller rather than a live bug.

Not included

True cross-worktree diffing (worktree A's files against worktree B's) is tracked separately in #23. It needs a model change, since DiffSide.WorkingTree is payload-free and ParsedCommandLine carries a single repo path.

Review findings addressed

Several findings arrived in review bodies rather than as inline comments. GitHub has no resolve state for those, so they stay visible at the top of the conversation and read as open forever. Tracking them here, since there is nowhere else to mark them done.

Finding	Fixed in
ResolveCommonDirectory returned an unnormalized path from its catch block	a36c3a2
WorktreePicker comment claimed EnsureLoadedAsync swallows failures	a36c3a2
Picker could stay empty after a mid-flight repo-path change	a1844b5
No service-level coverage for worktree label capture	a1844b5
HasAlternativeWorktrees mis-reported a bare hub with one linked worktree	a1844b5
Empty-state message rendered alongside "Loading..."	a1844b5
StackPanel denied the popup's ScrollViewer a viewport	a1844b5
Synchronous filesystem probe ran on the WPF dispatcher	51e83ee
Stash form could leave a newly selected worktree permanently unloaded	96ca109

All inline review threads are replied to and resolved.

Verification

• dotnet build -c Release clean, 0 warnings.
• dotnet test: 1644 passed, 1 skipped. The 1608 pre-existing tests pass unchanged.
• Smoke-tested the built exe against a linked worktree to confirm the new XAML loads.
• The three concurrency tests were re-run repeatedly to confirm determinism, after one was found racy and rewritten.

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Session

• AI-Local-Session: 80032277-bc2c-4613-a0d3-72ad20d3621b
• AI-Cloud-Session: b6d4fbf2-3954-4138-a5fb-aa9eaecbff64

[Copilot]

🟡 Changes recommended

There are a couple of correctness/documentation issues in new worktree-related helpers that should be addressed before approval.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds first-class Git worktree awareness to DiffViewer’s UI by persisting stable worktree/repository labels for recents, appending worktree names to the window title, and enabling switching the current diff or new-diff dialog to a different checkout via a shared worktree picker control and view-model.

Changes:

• Persist repository/worktree display labels into recents and surface them in the recents dropdown and window title
• Add a reusable Worktree picker popup and integrate it into the New Diff dialog and the context bar
• Extract local-repo-path handling into LocalRepoFormViewModelBase and introduce a LibGit2-based worktree enumerator plus test coverage

File summaries

File	Description
DiffViewer/Views/WorktreePicker.xaml.cs	Adds code-behind wiring for the worktree picker popup
DiffViewer/Views/WorktreePicker.xaml	Defines the worktree picker popup UI and row styling
DiffViewer/Views/RepoPathField.xaml.cs	Adds browse-folder behavior for the shared repo-path field control
DiffViewer/Views/RepoPathField.xaml	Introduces reusable repo-path field with Browse and Worktree toggle
DiffViewer/Views/RecentsBarView.xaml	Adds a context-bar Worktree switch toggle and embeds WorktreePicker
DiffViewer/Views/NewDiffDialog.xaml.cs	Removes repo-path browse logic now handled by RepoPathField
DiffViewer/Views/NewDiffDialog.xaml	Replaces repeated repo-path grids with the shared RepoPathField control
DiffViewer/ViewModels/WorktreePickerViewModel.cs	Adds VM to enumerate/pick worktrees and write back selected path
DiffViewer/ViewModels/WorkingTreeVsHeadFormViewModel.cs	Moves to LocalRepoFormViewModelBase for shared repo-path + picker support
DiffViewer/ViewModels/WorkingTreeVsCommitFormViewModel.cs	Moves to LocalRepoFormViewModelBase and re-points ref picker on repo resolution
DiffViewer/ViewModels/ViewStashFormViewModel.cs	Moves to LocalRepoFormViewModelBase and keeps stash enumeration behavior
DiffViewer/ViewModels/RecentContextsViewModel.cs	Adds worktree switcher integration via WorktreePickerViewModel
DiffViewer/ViewModels/NewDiffDialogViewModel.cs	Wires IGitWorktreeEnumerator into form dependencies
DiffViewer/ViewModels/MainViewModel.cs	Builds window title with optional worktree name; wires worktree switching
DiffViewer/ViewModels/LocalRepoFormViewModelBase.cs	Extracts local repo-path canonicalization, validation deferral, and worktree picker
DiffViewer/ViewModels/CommitVsCommitFormViewModel.cs	Moves to LocalRepoFormViewModelBase and syncs both ref pickers on repo resolution
DiffViewer/ViewModels/BranchVsMergeBaseFormViewModel.cs	Moves to LocalRepoFormViewModelBase and keeps merge-base seeding behavior
DiffViewer/Utility/GitWorktreeLayout.cs	Adds libgit2-free worktree layout reasoning and label derivation
DiffViewer/Services/RepositoryService.cs	Extends RepositoryShape with common-git-dir + worktree name data
DiffViewer/Services/RecentsJsonSerializer.cs	Serializes/deserializes repository/worktree label metadata for recents
DiffViewer/Services/RecentContextsService.cs	Captures worktree labels at record time via GitWorktreeLayout
DiffViewer/Services/NewDiffDialogHost.cs	Supplies worktree enumerator to the New Diff dialog VM
DiffViewer/Services/LibGit2GitWorktreeEnumerator.cs	Implements worktree enumeration with libgit2 quirks handled
DiffViewer/Services/IGitWorktreeEnumerator.cs	Introduces worktree enumeration interface and WorktreeEntry model
DiffViewer/Services/IDiffModeProvider.cs	Adds worktree enumerator to FormDependencies
DiffViewer/Models/RepositoryShape.cs	Adds CommonGitDirectory and WorktreeName and computed IsLinkedWorktree
DiffViewer/Models/RecentLaunchContext.cs	Adds repository/worktree label fields to recent context rows
DiffViewer/CompositionRoot.cs	Wires worktree enumerator into MainViewModel construction
DiffViewer/AppServices.cs	Adds optional WorktreeEnumerator to app service bundle
DiffViewer/App.xaml.cs	Instantiates and wires the libgit2 worktree enumerator into services
DiffViewer.Tests/ViewModels/WorktreePickerViewModelTests.cs	Adds unit tests for worktree picker VM behavior
DiffViewer.Tests/ViewModels/ViewStashFormViewModelTests.cs	Updates dependencies to include a stub worktree enumerator
DiffViewer.Tests/ViewModels/NewDiffFormViewModelTests.cs	Updates dependencies to include a stub worktree enumerator
DiffViewer.Tests/ViewModels/NewDiffDialogViewModelTests.cs	Updates VM construction for required worktree enumerator
DiffViewer.Tests/ViewModels/MainViewModelWindowTitleTests.cs	Adds tests for window title formatting with and without worktree name
DiffViewer.Tests/ViewModels/MainViewModelKeyboardShortcutTests.cs	Updates RepositoryShape construction for new required fields
DiffViewer.Tests/ViewModels/MainViewModelContextMenuTests.cs	Updates RepositoryShape construction for new required fields
DiffViewer.Tests/ViewModels/MainViewModelCommitMetadataTests.cs	Updates RepositoryShape construction for new required fields
DiffViewer.Tests/ViewModels/DiffPaneViewModelTests.cs	Updates RepositoryShape construction for new required fields
DiffViewer.Tests/ViewModels/BranchVsMergeBaseFormViewModelTests.cs	Updates dependencies to include a stub worktree enumerator
DiffViewer.Tests/Utility/GitWorktreeLayoutTests.cs	Adds tests for pure worktree layout reasoning and label derivation
DiffViewer.Tests/StubWorktreeEnumerator.cs	Adds test double for worktree enumeration
DiffViewer.Tests/Services/TempRepo.cs	Adds helpers for creating and cleaning up real linked worktrees in tests
DiffViewer.Tests/Services/RepositoryServiceTests.cs	Adds coverage for shape worktree name and common-git-dir invariants
DiffViewer.Tests/Services/PreDiffPassTests.cs	Updates RepositoryShape construction for new required fields
DiffViewer.Tests/Services/NewDiffDialogHostTests.cs	Updates host construction for required worktree enumerator
DiffViewer.Tests/Services/LibGit2GitWorktreeEnumeratorTests.cs	Adds behavioral tests for main and prunable worktrees enumeration
DiffViewer.Tests/RecentContexts/RecentsJsonSerializerTests.cs	Adds tests for label round-trip and backward compatibility
DiffViewer.Tests/RecentContexts/RecentContextsViewModelWorktreeTests.cs	Adds tests for context-bar worktree switching behavior
DiffViewer.Tests/RecentContexts/RecentContextItemTests.cs	Adds tests for new title formatting with repository/worktree labels
CHANGELOG.md	Documents worktree legibility and switching under Unreleased

Review details

• Files reviewed: 51/51 changed files
• Comments generated: 2
• Review effort level: Lite

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[Copilot]

🟡 Changes recommended

Worktree picker loading, bare-repository detection, and popup scrolling issues remain, and label persistence lacks service-level coverage.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

Suppressed comments (3)

Previously missed (3) — in code that hasn't changed since the last review.

DiffViewer/ViewModels/WorktreePickerViewModel.cs:84

• This assumes every enumeration contains a main entry, but the enumerator intentionally omits it for a bare hub. When a bare repository has exactly one linked worktree, that row is an available alternative yet this returns false and the popup reports that no other worktrees exist. Base the result on whether any entry is not current instead of the total count.
  DiffViewer/Views/WorktreePicker.xaml:60
• A vertical StackPanel measures this ScrollViewer with unbounded height, so the border's MaxHeight does not establish a viewport and the scrollbar will not make lower rows reachable when many worktrees exist. Use a constrained Grid row for the list or give the ScrollViewer an explicit maximum height.
  DiffViewer/Views/WorktreePicker.xaml:104
• HasAlternativeWorktrees is false before the asynchronous load finishes, so this empty-state message is visible at the same time as “Loading…”. Gate it on IsLoaded as well, so users are not told that no alternatives exist before enumeration has produced a result.

• Files reviewed: 51/51 changed files
• Comments generated: 2
• Review effort level: Balanced

[Copilot]

🔵 Needs a closer look

Synchronous filesystem probing can block the WPF UI thread during recent-context recording.

Review details

Suppressed comments (1)

Previously missed (1) — in code that hasn't changed since the last review.

DiffViewer/Services/RecentContextsService.cs:92

• This label lookup performs synchronous filesystem probes before RecordLaunchAsync has necessarily yielded. In the normal uncontended case, the coordinator calls this on the UI thread and WaitAsync completes synchronously, so an offline or slow network repository path can freeze the window during launch. Please move the probe to a background task, consistent with the repository's rule that filesystem I/O must not block WPF's dispatcher.

• Files reviewed: 52/52 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[geevensingh]

Synchronous filesystem probing can block the WPF UI thread during recent-context recording.
This label lookup performs synchronous filesystem probes before RecordLaunchAsync has necessarily yielded.

Confirmed and fixed in 51e83ee. The analysis was exactly right, including the subtle part: _gate.WaitAsync returns an already-completed task when uncontended, so the await never suspends and everything up to the first real suspension stayed on the dispatcher. All three TryRecordAsync call sites in MainWindowCoordinator await with ConfigureAwait(true) from the UI thread during a context swap, so the exposure is real rather than theoretical — a repo on a dead UNC mount would have blocked Directory.Exists until SMB timed out, freezing the window mid-launch.

The probe now runs through a runner defaulting to Task.Run, awaited before the gate is taken. That both guarantees the method yields before touching the file system and shortens the critical section, since the probe only depends on the repo path and needs no lock.

The runner is an internal seam mirroring IRootScanRunner in LocalRepoLocator, which exists for the same reason. Two tests cover it: one asserting the probe lands on a different thread than the caller, one asserting nothing is probed synchronously before the method returns. Both fail if the call is ever inlined back.

Noting for the record that this arrived as a review-body finding with no inline thread, so there is nothing to resolve on this one — replying here instead so the fix is on the record.

[Copilot]

🔵 Needs a closer look

Mid-flight stash enumeration can prevent the selected worktree’s stash list from ever loading.

Review details

Suppressed comments (1)

Previously missed (1) — in code that hasn't changed since the last review.

DiffViewer/ViewModels/ViewStashFormViewModel.cs:71

• Switching the stash form to another worktree while its initial stash enumeration is still running can leave the new checkout permanently unloaded. OnRepoPathResolved starts a new enumeration, but EnumerateStashesAsync immediately returns while IsLoading is true; the old request then detects the changed path and drops its result, with nothing retrying the new path. Coalesce or loop stale loads as WorktreePickerViewModel.EnsureLoadedAsync does, and cover the mid-flight path-change sequence.

• Files reviewed: 52/52 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[geevensingh]

Mid-flight stash enumeration can prevent the selected worktree's stash list from ever loading.

Confirmed and fixed in 96ca109. This is the same defect you found in WorktreePickerViewModel, in the other view-model that loads on a repo-path change — EnumerateStashesAsync turned away any caller arriving while IsLoading was true, then dropped its own result once the path had moved, leaving nothing to load the new checkout.

Worth noting why it belongs in this PR rather than being waved off as pre-existing: the guard and the stale-drop both predate these changes, but reaching them used to require typing a new path mid-enumeration. The worktree picker this PR adds turns that into a single click, so the change is what makes the bug readily reachable.

Fixed the same way as the picker — loop on a stale result instead of discarding it — with a test that re-points the form from inside the enumerate runner and asserts both paths were enumerated and the new checkout's stashes landed.

Separately, while validating this I found that a test I added in the previous round, RecordLaunchAsync_YieldsBeforeProbingTheFileSystem, was itself racy: Task.Yield only schedules the continuation, so whether the probe had run by the assertion was a coin flip, and it failed on its second execution. Replaced with a TaskCompletionSource gate so the probe provably cannot have run at the point of the check. Re-ran the three concurrency tests three times to confirm they're stable.

This one also arrived as a review-body finding with no inline thread, so there's nothing to resolve — replying here to keep the fix on the record.