Workaround WindowsAppSDK#6394: copy project .pri into AOT publish output

[codemonkeychris]

Summary

Diagnoses and works around a single WindowsAppSDK NativeAOT publish bug that was masquerading as ~10 unrelated Reactor framework issues. Net result: 68 of 108 previously-skipped AOT selftest fixtures now pass, including all 60 NATIVE_CRASH entries. JIT baseline is unchanged at 735/735.

Root cause

microsoft/WindowsAppSDK#6394 (OPEN against 1.8 and 2.0): when publishing an unpackaged WinUI 3 app (WindowsPackageType=None) with PublishAot=true, the WindowsAppSDK build pipeline generates $(AssemblyName).pri into $(TargetDir) but does not copy it into $(PublishDir). The PRI is wired into publish only behind <Condition>'$(AppxPackage)' == 'true'</Condition>, so unpackaged apps fall through. The same gap applies to .xbf files.

This one missing file produced what looked like several independent failures:

Symptom	Real cause
TabView ctor throws FileNotFoundException from ResourceAccessor::GetLocalizedStringResource	MRT can't find the PRI
NavigationView template-apply throws the same exception (caught by Reactor's error boundary)	MRT can't find the PRI
Application.Current.Resources loads empty (0/0/0)	Application.LoadComponent cascades through MRT for type lookups
ThemeRef.Resolve(...) returns null for every key	XamlControlsResources was empty, see above

Confirmed by literally copying Reactor.AppTests.Host.pri into the publish folder and re-running: every symptom resolved in one shot.

Fix

Small post-Publish MSBuild target in tests/Reactor.AppTests.Host/Reactor.AppTests.Host.csproj that copies the PRI (and any .xbf files) into the publish output:

<Target Name="_CopyWinUIResourcesForAot" AfterTargets="Publish"
        Condition="'$(PublishAot)' == 'true' and '$(AppxPackage)' != 'true'">
  <ItemGroup>
    <_WinUIResourcesForAot Include="$(OutputPath)**\*.xbf" />
    <_WinUIResourcesForAot Include="$(OutputPath)$(AssemblyName).pri"
                           Condition="Exists('$(OutputPath)$(AssemblyName).pri')" />
  </ItemGroup>
  <Copy SourceFiles="@(_WinUIResourcesForAot)"
        DestinationFiles="@(_WinUIResourcesForAot->'$(PublishDir)%(RecursiveDir)%(Filename)%(Extension)')"
        SkipUnchangedFiles="true" />
</Target>

Scoped to the test host since it's currently the only AOT-published exe in the repo. To be removed once WindowsAppSDK#6394 ships.

The Reactor library itself is untouched; it remains AOT-clean.

Skip-list cleanup (SelfTestRunner.DefaultAotSkipPatterns)

108 → 40 entries. Every NATIVE_CRASH skip is gone. The remaining 40 are ASSERT_FAIL in pre-acknowledged reflection-heavy subsystems:

• Devtools / MCP (27): JSON-RPC tool-discovery reflection — documented in docs/aot-support.md as not-yet-AOT-clean.
• PropertyGrid auto-discovery (9): walks user types via reflection — documented as not-yet-AOT-clean.
• Other (4): ControlUpdate_Collections, CoreCov2_UseObservableTreeHook (both pre-existing assertion bugs), Issue142_CustomControlPrivateDp_Renders, Issue142_ThirdPartyControlPrivateDp_Renders (need a hand-written IXamlMetadataProvider since the XAML compiler only emits one when a project has a .xaml file).

Verification

Suite	Result
JIT selftest (Reactor.AppTests.Host --self-test)	735/735 fixtures, 2614 ok, 0 not_ok, 0 bail
AOT selftest (with workaround + new skip list)	735 plan, 695 ran, 40 skipped, 0 not_ok, 0 bail
Reactor.Tests unit suite	Passed 8381, Failed 0, Skipped 46 (Yoga generated, pre-existing)
Reactor.csproj library build	0 errors, 0 IL2*/IL3* warnings (library remains AOT-clean)

Files changed

• tests/Reactor.AppTests.Host/Reactor.AppTests.Host.csproj — adds _CopyWinUIResourcesForAot MSBuild target.
• tests/Reactor.AppTests.Host/SelfTest/SelfTestRunner.cs — DefaultAotSkipPatterns shrinks 108 → 40 with new comments grouping the remaining entries by root cause.
• docs/aot-support.md — new "Required publish-time workarounds" section explaining #6394; works/does-not-work tables updated to reflect that built-in WinUI controls and ThemeRef.Resolve work under AOT with the workaround in place.

[codemonkeychris]

CI failure is unrelated — the Unit Tests job hit a known-flaky threading test:

Failed Microsoft.UI.Reactor.Tests.Core.UseInfiniteResourceThreadingTests.Refresh_During_InFlight_Cancels_And_Restarts
  Assert.Equal() Failure: Values differ
  Expected: 0
  Actual:   1
  at AssertNoUnobserved() at line 36

The assertion is TaskScheduler.UnobservedTaskException count after GC.Collect(); under CI load a late task completion can race the disposal/swap of the inner InfiniteResource, producing one unobserved exception.

Evidence this is pre-existing flake, not my PR:

• This PR touches only tests/Reactor.AppTests.Host/Reactor.AppTests.Host.csproj, tests/Reactor.AppTests.Host/SelfTest/SelfTestRunner.cs, and docs/aot-support.md. None of those reach src/Reactor/Core/InfiniteResource.cs or its hook.
• The most recent failed main CI run (3c3bed9e) failed with a sibling test in the same family (UseResourceThreadingTests.Fetcher_Observed_Cancellation_Is_Silent_Not_Error) hitting the same AssertNoUnobserved pattern.
• Local run: dotnet test ... --filter UseInfiniteResourceThreadingTests.Refresh_During_InFlight_Cancels_And_Restarts → Passed! Failed: 0, Passed: 1.

Reran the failed job.

[codemonkeychris]

Updated with a targeted fix for the failing unit test.

Root cause

The failures were a pre-existing flake in the Use*ThreadingTests family, exposed (but not caused) by this PR. All three test classes —

• ests/Reactor.Tests/Core/UseInfiniteResourceThreadingTests.cs
• ests/Reactor.Tests/Core/UseMutationThreadingTests.cs
• ests/Reactor.Tests/Core/UseResourceThreadingTests.cs

— subscribe to the process-wide TaskScheduler.UnobservedTaskException event in their constructor and assert _unobserved == 0 at the end of every test. With xUnit's default parallel test scheduling, the three classes raced each other: an unobserved Task exception from one test fired the event handler in another, breaking the invariant.

Evidence the failures are unrelated to this PR's AOT changes (three different tests in the family across three different runs, including a failure on main):

Run	Commit	Failing test
Main CI 26179835194	3c3bed9	UseResourceThreadingTests.Fetcher_Observed_Cancellation_Is_Silent_Not_Error
PR #360 original	42160a8	UseInfiniteResourceThreadingTests.Refresh_During_InFlight_Cancels_And_Restarts
PR #360 rerun	42160a8	UseMutationThreadingTests.Unmount_Cancels_Pending_OnError_Does_Not_Fire

Fix

Tag all three classes with [Collection("UnobservedTaskException")] — xUnit serializes classes sharing a collection name, so they no longer run concurrently. Same convention as the existing [Collection("ConsoleTests")] group used for Console.Out/Error-mutating tests.

Verified locally: 10x stress run of all 19 threading tests, 0 failures.

Also rebased on current main to pull in #356 + #357.