DevTools expects to receive an animation mutation (via MutationObserver) when an animation is removed because of a class change, but this appears to not happen with Stylo. This leads to timeouts in tests like: * devtools/server/tests/browser/browser_animation_emitMutations.js : https://treeherder.mozilla.org/logviewer.html#?job_id=105301401&repo=try&lineNumber=3470
Created attachment 8875921 [details] An example Interesting. Initially I thought we don't cancel the animation when the corresponding class name is removed, but as far as I can see, the animation is cancelled.
(In reply to Hiroyuki Ikezoe (:hiro) from comment #1) > Interesting. Initially I thought we don't cancel the animation when the > corresponding class name is removed, but as far as I can see, the animation > is cancelled. There are definitely some cases for transitions where we don't cancel them, but for animations we should.
This may be the cause of the failures in dom/animation/test/chrome/test_animation_observers_a?sync.html too (a test I plan to look into later today).
The test page used by this DevTools test is: http://searchfox.org/mozilla-central/rev/a798ee4fc323f9387b7576dbed177859d29d09b7/devtools/server/tests/browser/animation.html Specifically, it appears to change the node with the class "not-animated" to "multiple-animations" (which seems to work) and then later back to "not-animated" (which is the step where removed animations are missed). The test page seems to contain a mix of both animations and transitions together.