MouseTrailer events fire off the Mozilla event queue, so we don't reenter drag code. Patch by me and ere, r=ere on the parts I wrote, r+sr=me on the parts ere wrote. This is wallpaper; we need a real fix here. git-svn-id: svn://10.0.0.236/trunk@169948 18797224-902f-48f8-a5cc-f745e15eee43