Screensaver sometimes leaks contents of desktop while locked

Binary package hint: gnome-screensaver

Occasionally when coming back to my computer after having locked the screen, I tap the shift key to present the unlock dialog box to enter my password, and very briefly the contents of my desktop flashes on the screen before the screen repaints black with the password prompt dialog. Inevitably if I then immediately relock the system and try again, it does not leak anything.

I'm using:
- Jaunty with latest updates
- The "blank screen" screensaver
- fglrx
- compiz with mostly the same options as "normal visual effects." "Unredirect fullscreen windows" is enabled (which I'll now disable and see if that makes any difference).

I don't know if the problem is with gnome-screensaver specifically, or compiz. Unfortunately I can't consistently reproduce this problem; it only happens occasionally.