manual-tests/bugzilla-6821.html is failing: a yellow bar remains above the green box and doesn't change to green until you move the mouse. The most likely candidate for the regression is r15544, fox for bug 8272, which probably changed something about the fake mouse moved even that is dispatched when the hovered element detaches.
I think the fix is to just use PlatformMouseEvent(PlatformMouseEvent::currentEvent) in FrameView::hoverTimerFired.
Created attachment 10591 [details] proposed fix
Comment on attachment 10591 [details] proposed fix ggaren probably knows the most about this code. But this looks sane to me.
Comment on attachment 10591 [details] proposed fix r=me
Committed revision 16525.