We were mistakenly wrapping any Event or subclass as a WebKitDOMEvent, but we need to return the proper GObject type whenever appropriate (ie, a WebKitDOMMouseEvent for MouseEvent).
Created attachment 64567 [details] wrapevents.diff Patch.
Comment on attachment 64567 [details] wrapevents.diff OK!
Comment on attachment 64567 [details] wrapevents.diff Landed in r65497.
Closing bug.