In the attached Mochitest a 10s animation is set up with an iteration count of 0.3 making it run for only 3s. At t=4s, the animation has finished and should no longer be running on the compositor thread but this test shows that it still is.

Actually this is specific to opacity animations. With transforms we have a flag indicating if a transform was set by animations or not but we don't with animation. Hence, if we have a layer and it has opacity we'll end up returning a value from DOMWindowUtils.getOMTAStyle but that doesn't mean it's being animated. The test cases fails because it's broken. It doesn't fetch the animated state after waiting for paints to be flushed.