fix(quickfiler-test): route WpfUiDispatcherTests static swap through the shared fixture

[drmoisan]

Summary

• Routes the process-wide UtilitiesCS.Threading.UiThread._dispatcher swap in QuickFiler.Test/Controllers/WpfUiDispatcherTests.cs through the shared UiThreadDispatcherFixture transaction introduced by Bug: uithread-dispatcher-static-swap-no-restore #493, removing the last ungated mutator of that static inside QuickFiler.Test.
• Replaces raw reflection (GetField / SetValue) and an unconditional finally restore with UiThreadDispatcherFixture.BeginTransactionAsync(), transaction.Install(dispatcher), and transaction.Dispose(), which restores by ReferenceEquals compare-then-write and only then releases the gate.
• Behavior is unchanged: Invoke, InvokeAsync, and BeginInvoke are still each asserted to execute their delegate on the dispatcher's own thread, and Construction_YieldsAnIUiDispatcher is untouched.
• Exactly one .cs file changes. No production source, no UtilitiesCS/ path, and no UtilitiesCS.Test/ path is modified.
• Feature review returned PASS with zero blocking findings; all seven acceptance criteria were independently re-verified.

Why

WpfUiDispatcherTests.cs predates the shared fixture, so it had no gated path available when it was written. After #493 landed, every other mutation of that static from this assembly's owned files holds FieldLock for the whole read-modify-write, leaving this one call site as the remaining ungated writer.

Unlike the originating #493 defect, this site did restore the previous value, so it was a lesser and distinct concern rather than a recurrence of the no-restore bug. What it lacked was participation in the lock protocol: it never acquired UiThreadDispatcherFixture.FieldLock, so its read-modify-write could interleave with a fixture transaction, and its restore was an unconditional write rather than the fixture's conditional compare-then-write.

This was recorded as accepted residual risk R-1 of #493, whose Rollout & Follow-up item 3 asked that it be promoted as its own small issue once the shared fixture existed.

What Changed

Test source (1 file)

• QuickFiler.Test/Controllers/WpfUiDispatcherTests.cs — reflection block and unconditional restore removed; using System.Reflection; and using UtilitiesCS; dropped; the method is now async Task with [Timeout(GateTimeoutMs)] and a private const int GateTimeoutMs = 60000; class field matching the sibling Bug: uithread-dispatcher-static-swap-no-restore #493 regression tests; nested try/finally so transaction.Dispose() runs before QfcItemControllerTestSupport.ShutdownDispatcher(dispatcher).

Evidence and audits (50 files)

• Phase 0 baseline, Phase 1 regression-testing, and Phase 2 QA-gate artifacts for every plan task, plus the fail-before exception dossier and the three feature-review audit artifacts.

Agent memory (17 files)

• Lessons recorded during preparation, execution, and review.

Architecture / How It Fits Together

UiThreadDispatcherFixture (QuickFiler.Test/Controllers/QfcItemController.UiThreadDispatcherFixture.cs) is the single owner of every mutation of UiThread._dispatcher made from this assembly's owned files, and #493's AC-4 gates that uniqueness. It guards two concerns with two locks: TransactionGate serialises whole transactions, and FieldLock makes each read-modify-write of the static atomic. Lock order is TransactionGate then FieldLock, never the reverse, so no cycle exists.

The test now acquires the gate, installs the running dispatcher through the returned UiThreadDispatcherTransaction, and disposes it to restore. No second reflection lookup is introduced.

Verification

Completed

• CSharpier check across the tree: exit 0, 1566 files, no drift.
• Analyzer rebuild (/t:Rebuild, EnableNETAnalyzers, EnforceCodeStyleInBuild): exit 0, 0 Error(s), warning count equal to the Phase 0 baseline.
• Nullable rebuild (/t:Rebuild, TreatWarningsAsErrors): exit 0, 0 Error(s), warning count equal to baseline.
• Scoped run of WpfUiDispatcherTests: 2/2 passing.
• Full QuickFiler.Test.dll under the repo runsettings (Scope=ClassLevel, Workers=0): 1285/1285 passing, no regression against the 1285 baseline.
• Coverage suite: 6925/6925 passing. Repo-wide first-party coverage 85.373% line (54964/64381) and 79.694% branch (23215/29130), both above the 85%/75% floors.
• Feature review re-ran the five dispatcher-touching classes together at ClassLevel with 24 workers: 47/47 passing.

Recommended

• dotnet tool run csharpier check .
• msbuild TaskMaster.sln /t:Rebuild /m /p:Configuration=Debug "/p:Platform=Any CPU" /p:EnableNETAnalyzers=true /p:EnforceCodeStyleInBuild=true
• msbuild TaskMaster.sln /t:Rebuild /m /p:Configuration=Debug "/p:Platform=Any CPU" /p:TreatWarningsAsErrors=true
• vstest.console.exe QuickFiler.Test\bin\Debug\QuickFiler.Test.dll /Settings:scripts\vscode\TaskMaster.cli.runsettings /InIsolation /TestCaseFilter:"TestCategory!=LiveOutlook"

A green run under class-level parallelization does not prove the race is eliminated; it shows only that the gated path is stable under that scope. The defect was a latent ordering hazard, dormant under CI settings, so no deterministic red run was available. A fail-before exception dossier records why and names the substitute evidence.

Backward Compatibility / Migration Notes

None. The change is confined to one test method body. No public API, no production code, and no project file is altered.

Risks and Mitigations

• The rewritten test now contends on TransactionGate with the Bug: uithread-dispatcher-static-swap-no-restore #493 regression tests. Measured harmless: 47/47 pass in 1.45 s with 24 workers. The 60-second [Timeout] converts a genuine deadlock into a test failure rather than a hung run.
• Out-of-scope mutators remain. UtilitiesCS.Test/Threading/{ProgressTracker_Tests,ProgressTrackerAsync_Tests,IdleAsyncQueue_Tests}.cs still mutate the same process-wide static. No test-side lock inside QuickFiler.Test can reach them. These are accepted residual risk R-2 of Bug: uithread-dispatcher-static-swap-no-restore #493 and overlap the still-open Bug: uithread-dispatcher-null-race-progresstrackerasync #584. This PR deliberately does not touch them.

Review Guide

1. QuickFiler.Test/Controllers/WpfUiDispatcherTests.cs — the entire functional change.
2. docs/features/active/2026-08-27-wpfuidispatchertests-ungated-static-swap-648/code-review.2026-09-01T14-06.md — including the assessment of the one deviation from the plan.
3. The remaining evidence files are mechanical gate records.

One deviation from the approved plan, accepted on review. Plan task P1-T2 directed the gate acquisition as a single expression, await UiThreadDispatcherFixture.BeginTransactionAsync().ConfigureAwait(false). At the 16-column indent that line exceeds CSharpier's 100-column default, and CSharpier reprints it as a three-line member chain, which would have made P1-T6's single-line token assertion unsatisfiable for any executor who followed P1-T2 literally. The reviewer reproduced this against the manifest-pinned CSharpier 1.2.6 rather than accepting the claim. The delivered form binds the task to a local first; it is the same invocation, the same task, and the same ConfigureAwait(false).

Follow-ups

Recorded here rather than opened as issues, because acceptance criterion AC-6 restricts this branch's diff to a named path set and a promotion would breach it. They are reported separately for consolidated filing.

• Two plan-authoring defects. P1-T2 and P1-T6 were mutually unsatisfiable as authored (above). Separately, P2-T3's condition "no diagnostic names Controllers\WpfUiDispatcherTests.cs" matches the csc.exe command line and its response-file echo on a clean build, so an unrestricted log search returns false hits and the condition is unsatisfiable unless first filtered to diagnostic lines. Reachability of both: certain.
• Raw Cobertura remains reachable in branch history. Two ~10.6 MB reports were committed before being converted to package-level JaCoCo projections, so the working tree no longer carries them but the blobs stay reachable. Squash-merging this PR drops both with no history rewrite. The projections reconcile exactly to the source root counters. Note this is a pre-existing repository-wide pattern: origin/main currently carries 281 .cobertura.xml files.
• Evidence Timestamp: fields are synthetic. Several artifacts carry stamps inconsistent with their own quoted build banners and commit dates. Review confirmed the gates did in fact run, via recorded elapsed times and artifact mtimes; the stamps simply cannot establish inter-gate ordering.
• .globalconfig is referenced twice in CLAUDE.md but does not exist. .editorconfig is the actual analyzer severity source. Documentation-only.

PR Context Bundle Errors

The generated context bundle at artifacts/pr_context.summary.txt is unreliable for this PR and its auto-close list was not used:

• It reports "GitHub CLI unavailable: GitHub CLI (gh) is not installed." This is false; gh is installed and working in this checkout and was used to verify every issue number below.
• Its author-asserted auto-close list contained 18 entries scraped from prose. Nine are not issue numbers at all (#AC-1 through #AC-7, #ISO-8601, #SHA-256). Of the nine real numbers, seven are unrelated and already closed (Build a WinForms message-pump test seam (Application.Run() background thread) to unblock 9 QfcItemController orchestration members #230, Bug: timeouttask-runwithtimeout-exception-type-mismatch #285, Bug: storewrapper-dialog-imprecise-for-genuine-failure #287, Bug: uithread-dispatcher-static-swap-no-restore #493, Bug: qfc-unsynchronized-undo-handoff-after-batch-move #633, Bug: qfc-metrics-flush-writes-empty-session-file #646, Bug: fileio2-write-retry-reports-success-on-final-failure #647), and one — Bug: uithread-dispatcher-null-race-progresstrackerasync #584 — is open and is explicitly out of scope for this change.

Each number was verified directly against GitHub. Only #648 is closed by this PR.

GitHub Auto-close

• Closes Bug: wpfuidispatchertests-ungated-static-swap #648