Bug: wpf-dispatcher-yield-test-order-dependent

[drmoisan]

• Work Mode: minor-audit

Summary

UtilitiesCS.Test\OutlookObjects\Folder\WpfDispatcherYieldTests.YieldAsync_WithoutDispatcher_RemainsStrict fails intermittently on the full-suite run. The test asserts that WpfDispatcherYield.YieldAsync throws InvalidOperationException when no WPF Dispatcher is associated with the current thread, but under class-level parallel execution the pooled worker thread it lands on may already have a Dispatcher attached by an earlier test, so no exception is thrown and the assertion fails.

Environment

• OS/version: Windows 11
• Runtime: .NET Framework 4.8.1
• Command/flags used: pwsh -File scripts/vscode/Invoke-MSTestWithCoverage.ps1 -Configuration Debug (full-suite run, class-level parallelization, 24 workers)
• Data source or fixture: none

Steps to Reproduce

1. Build the solution in Debug.
2. Run the full MSTest suite across all *.Test.dll assemblies with parallelization enabled, as CI and the local coverage script both do.
3. Repeat the run.

Observed across two consecutive baseline runs on bug/ribbon-engine-readiness-guard-503 at merge-base 003c5715:

• Run 1: Total tests: 6293, Passed: 6291, Failed: 2
• Run 2: Failed: 1 - YieldAsync_WithoutDispatcher_RemainsStrict

The failure count changed between runs with no intervening code change, which establishes non-determinism rather than a deterministic regression.

Expected Behavior

The test produces the same result on every run regardless of which other tests executed first or which pooled thread it runs on, per .claude/rules/general-unit-test.md Core Principles 1 (Independence) and 4 (Determinism).

Actual Behavior

[TestMethod]
public async Task YieldAsync_WithoutDispatcher_RemainsStrict()
{
    var dispatcherYield = new WpfDispatcherYield();

    await dispatcherYield
        .Invoking(item => item.YieldAsync(CancellationToken.None))
        .Should()
        .ThrowAsync<InvalidOperationException>();
}

(UtilitiesCS.Test/OutlookObjects/Folder/WpfDispatcherYieldTests.cs:28-37.)

The "without dispatcher" precondition is an ambient property of the executing thread, not something the test arranges. Dispatcher.CurrentDispatcher creates and caches a dispatcher for the calling thread on first access, so any earlier test on the same pooled worker that touches a WPF dispatcher leaves one attached, and this test's precondition silently evaporates.

Logs / Screenshots

• Attached minimal logs or snippet
• Snippet:

  Failed YieldAsync_WithoutDispatcher_RemainsStrict [162 ms]
  Error Message:
     Failed: 1

Impact / Severity

• Blocker
• High
• Medium
• Low

The suite is not reliably green at baseline, which undermines every downstream quality gate: an agent or developer cannot distinguish "my change broke a test" from "the suite is flaky". It also produces spurious CI failures and encourages re-running until green, which is the failure mode the determinism rule exists to prevent.

Source

From: docs/features/potential/2026-08-08-wpf-dispatcher-yield-test-order-dependent.md