eventSender.mouseMoveTo() does not propagate a mouse moved event to the frame. This makes it difficult to test onmouseover/onmouseout and :hover (for the latter, I had to add a "click" to the test script in order for the mouse position to reach the frame). I am going to attach a patch.
Created attachment 6842 [details] Patch Note that mouse moved events will only be received following initial layout, since that's when WKSetNSWindowShouldPostEventNotifications is called.
Comment on attachment 6842 [details] Patch r=me