The following layout test is flaky on Mac Debug media/media-source/media-source-seek-redundant-append.html Probable cause: This test appears to have become flakey recently. We need to try and find the regression point. Flakiness Dashboard: https://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=media%2Fmedia-source%2Fmedia-source-seek-redundant-append.html
<rdar://problem/45647917>
Note this is a relatively recent test, introduced in r236656 (Oct 1 2018), very close to the first (flaky) timeout, so it's very likely that the problem has been there for very long before the test existed. The test has not had any failures in GTK or WPE so far, so it is probably an issue in the Mac backend rather than in platform-independent code.