Firefox 4.0b10 on Mac OS X 10.6 with Flash 10.2.152.14. I run into this with (paused) videos on YouTube from time to time. After switching back to a YouTube tab, the area where the video is will be unpainted or partially painted. Moving the mouse into the video region will cause parts to be repainted (due to the control bar sliding up), hitting play will repaint the rest. It _could_ be that this is a Flash bug and not Gecko, though I don't recall running into it until OOPP was landed on Mac. Unfortunately no steps to reproduce yet.
You need to log in before you can comment on or make changes to this bug.