this seems related to this bug: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+bug/416001
and the fix suggested in the link fixes the issue.
this seems related to this bug: https:/ /bugs.launchpad .net/ubuntu/ +source/ xserver- xorg-video- ati/+bug/ 416001
and the fix suggested in the link fixes the issue.