Convert tests to UIThreadTestMethod pattern

[Copilot]

Migrates tests from [TestMethod] with EnqueueAsync to [UIThreadTestMethod] pattern, adding explicit composition rendering waits for property propagation. This addresses CI failures on .NET Native UWP builds.

Tests Converted

1. ShouldConfigureGridSplitterAutomationPeer (ExampleSizerBaseTestClass.cs)

   • Replaced [TestMethod] + EnqueueAsync wrapper with [UIThreadTestMethod]
   • Added CompositionTargetHelper.ExecuteAfterCompositionRenderingAsync() after gridSplitter.Name and gridSplitter.SetValue() calls to ensure property changes propagate before assertions

2. ComplexAsyncLoadUIExampleTest (ExampleColorPickerTestClass.cs)

   • Replaced [TestMethod] + EnqueueAsync wrapper with [UIThreadTestMethod]
   • Added CompositionTargetHelper.ExecuteAfterCompositionRenderingAsync() after LoadTestContentAsync and UnloadTestContentAsync to ensure UI updates complete before assertions

Before/After

// Before
[TestMethod]
public async Task TestName()
{
    await EnqueueAsync(async () =>
    {
        await SomeUIOperation();
        // assertions
    });
}

// After
[UIThreadTestMethod]
public async Task TestName()
{
    await SomeUIOperation();
    await CompositionTargetHelper.ExecuteAfterCompositionRenderingAsync(() => { });
    // assertions
}

Pattern matches existing usage across 15+ test files in the codebase.

[!WARNING]

Original prompt

Convert the test ShouldConfigureGridSplitterAutomationPeer in components/Sizers/tests/ExampleSizerBaseTestClass.cs to use [UIThreadTestMethod] instead of [TestMethod] and remove usage of EnqueueAsync. After each assignment to gridSplitter.Name and gridSplitter.SetValue, add the line:
await CompositionTargetHelper.ExecuteAfterCompositionRenderingAsync(() => { });
This ensures test logic waits for property propagation before asserting. The test framework must support [UIThreadTestMethod].

Summary of changes:

• Use [UIThreadTestMethod] on the test method and remove EnqueueAsync wrapper.
• After setting gridSplitter.Name and gridSplitter.SetValue, insert await CompositionTargetHelper.ExecuteAfterCompositionRenderingAsync(() => { });
• Do not reference any GitHub issue in the PR, as per team direction and repository policy.

This pull request was created from Copilot chat.

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[michael-hawker]

Updated to run it again will see if it passes again or if fluke. If this does make a different then it at least gives us some insight that it could be this specific test pattern that somehow has issues... FYI @Arlodotexe

[michael-hawker]

(UWP CI fail on .NET Native)

[michael-hawker]

ColorPickerTests.ExampleColorPickerTestClass.ComplexAsyncLoadUIExampleTest is another one failing and uses the same pattern.

[michael-hawker]

@copilot apply the same [UIThreadTestMethod] fix to the ColorPickerTests.ExampleColorPickerTestClass.ComplexAsyncLoadUIExampleTest test in this same PR.

[Copilot]

@copilot apply the same [UIThreadTestMethod] fix to the ColorPickerTests.ExampleColorPickerTestClass.ComplexAsyncLoadUIExampleTest test in this same PR.

Applied the same fix to ComplexAsyncLoadUIExampleTest. The test now uses [UIThreadTestMethod] instead of [TestMethod] with EnqueueAsync, and includes composition rendering waits after LoadTestContentAsync and UnloadTestContentAsync to ensure UI updates complete before assertions. Changes in commit b6da782.