170113 2017-03-27 02:03:34 -0700 ASSERT(!needsLayout()) in FrameView::paintContents() when setting a CSS class during handling of a "loadedmetadata" event 2017-03-27 11:08:54 -0700 1 1 1 Unclassified WebKit Layout and Rendering WebKit Nightly Build Unspecified Unspecified NEW https://bugs.webkit.org/show_bug.cgi?id=169145 InRadar P2 Normal --- 1 graouts webkit-unassigned bfulgham simon.fraser webkit-bug-importer zalan oldest_to_newest 1291433 0 graouts 2017-03-27 02:03:34 -0700 A previous version of the patch for https://bugs.webkit.org/show_bug.cgi?id=169145 would infrequently hit `ASSERT(!needsLayout())` in FrameView::paintContents(). This was due to those three lines in _mediaDocumentHasMetadata() in Source/WebCore/Modules/modern-media-controls/media/media-document-controller.js: const media = this.mediaController.media; media.classList.add(this.mediaController.isAudio ? "audio" : "video"); media.classList.add(window.navigator.platform === "MacIntel" ? "mac" : window.navigator.platform); This code is called as a result of handling a "loadedmetadata" event, which is fired asynchronously. You can re-create the assertion by removing the enclosing requestAnimationFrame() call and running this command: run-webkit-tests --debug -1 --iterations=500 --exit-after-n-failures=1 fast/events/media-focus-in-standalone-media-document.html You should see DRT crash relatively quickly as this ASSERT is hit. 1291435 1 webkit-bug-importer 2017-03-27 02:05:37 -0700 <rdar://problem/31269350> 1291461 2 simon.fraser 2017-03-27 07:58:53 -0700 What's the full stack trace? 1291464 3 graouts 2017-03-27 08:07:11 -0700 https://build.webkit.org/results/Apple%20Sierra%20Debug%20WK1%20(Tests)/r213410%20(3609)/fast/events/media-focus-in-standalone-media-document-crash-log.txt 1291529 4 simon.fraser 2017-03-27 11:08:54 -0700 Ah, it's coming out of figDeferredNotificationDispatchFunction. This is tracked by rdar://problem/16622896.