The first failure that I see this on the dashboard was 09/12/20 at r266974, but is not relevant to what would have caused this.
The test was introduced at r266972 and has been flaky failure since then.
The test was expecting a fader margin: 50px;height: 200px;background-color: rgba(255, 255, 255, 0.5); but returned fader with larger width and position higher.
run-webkit-tests --exit-after-n-failures 1 --iterations 50 --debug compositing/animation/repaint-after-clearing-shared-backing.html
Regressions: Unexpected image-only failures (1)
compositing/animation/repaint-after-clearing-shared-backing.html [ ImageOnlyFailure ]
I have marked this test as image only failure while this issue is investigated.
Created attachment 413152 [details]
Committed r269367: <https://trac.webkit.org/changeset/269367>