Comment 3 for bug 1178000

From Upstream: https://bugzilla.gnome.org/show_bug.cgi?id=700594#c4

Owen Taylor [mutter developer] 2013-05-20 16:23:46 UTC
If everything works as expected after disabling and reenabling the display in
System Settings, then this is a bug in the X server, in gnome-settings-daemon,
or (less likely) in the Mesa GL drivers.

(In the case where the display isn't working, do 'xdpyinfo| grep dimensions' -
if the monitor's pixels show up there, then it might be a Mesa bug, but if they
don't show up, then that excludes Mesa and it's either an X or g-s-d bug)

From Upstream: https://bugzilla.gnome.org/show_bug.cgi?id=700594#c5

Richard Eames [reporter] 2013-05-20 16:55:09 UTC
I sometimes have to disable/reenable the monitor multiple times before I can
get it to work. There are three cases that I get when I enable/reenable the
monitor:
1 : Works as expected, I get a screen that spans all three monitors
2 : The screen is shifted by about half the width of the third monitor
3 : Similar to 3, except that the middle monitor displays corrupt graphics.

This is what I get on case 1, and before I've done the disable/reenable.
$ xdpyinfo |grep dimen
  dimensions: 5520x1080 pixels (1461x286 millimeters)

I also tested in Ubuntu Unity with the same results, so that rules out
gnome-shell.

What's my next step for ruling out Mesa / X / g-s-d?