(I'm a little curious why this hasn't been causing problems more often - my shot-in-the-dark guess is that usually we synthetically move the mouse often enough to prevent the screensaver from ever triggering, for most buckets of tests.)
Bug 1843878 Comment 11 Edit History
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(I'm a little curious why this hasn't been causing problems more often - my shot-in-the-dark guess is that usually we synthetically move the mouse often enough (in various tests towards the beginning and middle of each mochitest "bucket") to prevent the screensaver from ever triggering at least for the "buckets" of tests that include tests near the end that require focus and would fail if the screensaver had activated)