Please take a look at: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/476874/comments/4 it looks like a bug in gdm.
Please take a look at: https:/ /bugs.launchpad .net/ubuntu/ +source/ xserver- xorg-video- intel/+ bug/476874/ comments/ 4
it looks like a bug in gdm.