Comment 6 for bug 475428

Experienced the same issue on an older radeon card (radeon 8500) following an automated update of xserver-xorg-video-radeon (update manager suggestion). Solved the issue by adding some sections to the auto-configured xorg.conf. Notably changing to the older "XAA" AccelMethod appears to fix the issue:

Section "Device"
 Identifier "Radeon 8500"
 Driver "ati"
 BusID "PCI:1:0:0"
 Option "XAANoOffscreenPixmaps"
 Option "AccelMethod" "XAA"
 Option "EnablePageFlip" "true"
 Option "TripleBuffer" "true"
EndSection

Section "Monitor"
 Identifier "Configured Monitor"
EndSection

Section "Screen"
 Identifier "Default Screen"
 Monitor "Configured Monitor"
 Device "Radeon 8500"
EndSection