Thanks, I will try the patch!

In case somebody has my graphics card (or a HD 7790 or maybe others too??) I
found a definite and easy way to reproduce it:

1. Start a lightweight DE with e.g. openbox or xfwm4 (compositing disabled)
2. Install and start "compton" from a terminal
3. Play this youtube video from firefox with flashplayer:
http://www.youtube.com/watch?v=d-Cc6j2ZBwk (happens on others too, but this
video is definately affected)
4. The bug should occur in less than 1:30 minutes
5. If it doesn't occur the first time just reload

This should definitely trigger the bug.