Thanks for your bug report. Which video driver are you using? Do you use compiz and xgl by any chance? Which repositories do you have enabled? Does this happen with a new user as well? Do you have any interesting log messages in ~/.xsession-errors?
Thanks for your bug report. Which video driver are you using? Do you use compiz and xgl by any chance? Which repositories do you have enabled? Does this happen with a new user as well? Do you have any interesting log messages in ~/.xsession-errors?