Flaky test: LinkAll.LinkAllRegressionTest.Pasteboard_ImagesTest fails intermittently on shared UIPasteboard state

[mmitche]

Summary

LinkAll.LinkAllRegressionTest.Pasteboard_ImagesTest (in tests/linker/link all/LinkAllTest.cs) intermittently fails with:

Assert.That (UIPasteboard.General.Images.Length, Is.EqualTo (2), "b - length")
  Expected: 2
  But was:  1

Evidence

• Confirmed occurrence: build 15214743 (PR [msbuild] Add diagnostic logging for native reference resource resolution (#24564) #26563, MacCatalyst linker suite, T: linker_maccatalyst job), first attempt, 2026-09-04T14:50:02.638Z. Automatically recovered on stage retry (attempt 2 passed).
• 30-day AzDO Analytics query (test name Pasteboard_ImagesTest, 2026-08-15–2026-09-14): 22 failures out of 5,616 executions (~0.39% failure rate), recurring on multiple dates (08-29, 08-30, 09-01, 09-02, 09-03, 09-04, and 09-09).

Root cause hypothesis

UIPasteboard.General is a shared/global system resource. Parallel or back-to-back test execution can race on its contents, causing Images.Length to read a stale or partial value.

Related prior art

This is the same failure signature previously seen on the sibling test MonoTouchFixtures.UIKit.PasteboardTest.ImagesTest in tests/monotouch-test/UIKit/PasteboardTest.cs:

• Commit eafa2d1b (2024) added [Retry(10)] due to shared-clipboard races.
• Commit cba6e28f (2025, [monotouch-test] Ignore the PasteboardTest.ImagesTest. #24303) later added [Ignore(...)] because it still failed randomly even after 10 retries ("the clipboard sometimes has just one image for some reason").

LinkAll.LinkAllRegressionTest.Pasteboard_ImagesTest does not currently have a [Retry] or [Ignore] attribute, so this mitigation was not applied to this test copy.

Suggested next step

Apply the same mitigation used for the sibling test ([Retry] and/or [Ignore] with a tracking comment), or investigate making pasteboard clear/set operations deterministic per test, such as serializing tests that use the shared pasteboard.

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Filed from automated flakiness triage issue 32097719cadb4f4ca8a940ff91c6d6b9, revision 1, after human-review handoff and investigation. No code changes were made.