This is a deep upstream issue that requires knowledge of the ATI driver and Cairo, and their interaction. I believe it has been reported upstream (see the bugwatch on https://bugs.freedesktop.org/show_bug.cgi?id=5518 on the report) but there's not a lot of action there.
In Edgy, with newer drivers AND a newer Cairo, this problem is still evident. Perhaps someone who is subscribed here know who to nudge upstream to look at this? -- Cairo and ATI RENDER extension cause scrambled buttons in UbuntuLooks https://launchpad.net/bugs/34435 -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs
