Comment 4 for bug 36321

This happens to me, too, and I'd like to add how XGL is started exactly to make it better reproduceable:
A new xsession is created:
/usr/share/xsessions/xgl.desktop

containing:

[Desktop Entry]
Encoding=UTF-8
Name=XGl
Exec=/usr/bin/startxgl.sh
Icon=
Type=Application

where /usr/bin/startxgl.sh is the script which actually starts xgl and looks in a minimal case (not starting compiz withit) like this:

#!/bin/bash

Xgl -fullscreen :1 -ac -accel glx:pbuffer -accel xv:fbo & sleep 2 && DISPLAY=:1 gnome-session

This is a popular method to start XGL, as it leaves you the option to start a normal Xorg Session from GDM.

It is reported e.g.:
http://ubuntuforums.org/showthread.php?t=174233
http://compiz.net/viewtopic.php?pid=3645#p3645
http://tinyurl.com/lgjsz
http://tinyurl.com/n8vkk
(tiny as I don't want to break the layout...)