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[1] [1]: https://treeherder.mozilla.org/logviewer.html#?job_id=105301401&repo=try&lineNumber=3470

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.

This test passes for me, locally. (Despite a few assertions, which are bug 1189015.) Ryan, can you confirm that this test is passing on try in your next try run?

(In reply to Brian Birtles (:birtles) from comment #3) > 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 sync version of that test is passing for me too. The async version is failing, and I think that's covered by bug 1376248.

(In reply to Cameron McCormack (:heycam) from comment #5) > This test passes for me, locally. (Despite a few assertions, which are bug > 1189015.) Ryan, can you confirm that this test is passing on try in your > next try run? Looks like it is now passing on try and locally for me as well! :)