On Linux M-e10s, the dom/events/test/test_bug607464.html test seems to fail pretty frequently when I enable APZ. I saw this a while back but then it went away and now it's back. Anyway I can reproduce it locally (hopefully it's the same issue as on tryserver) so I might as well look into it. Initial investigation seems to indicate that the wheel events are getting synthesized by JS but the scroll event isn't coming out, so somewhere along the way the events are getting dropped.

Seems to be happening because the new window doesn't get painted before the wheel events are sent. The layer and hit-testing tree in the compositor don't even have the subframe and so the wheel events don't do anything. Waiting for an APZ flush before dispatching the wheel events seems to fix it.