I've tried all the solution I could find on the matter so far, but the error persists. My problem is that the login screen (both under gdm and lightdm) runs with the wrong resolution, even though all is fine when I log in. The error occurs solely when I have my HDMI cable connected to my other screen. The login screen resolution becomes 1024x768 (for my 1366x768 laptop screen) and mirrored on my screen, which is 1920x1080.

I've had this issue on version 12.04 (the last one before I upgraded to 13.04), but I got it fixed by adding the xrandr commands on the begining of the /etc/gdm/Init/Default file. This doesn't seem to work anymore.

I've also tried telling lightdm to run a script fixing the resolution with xrandr (by editing /etc/lightdm/lightdm.conf), but lightdm crashes, and I'm forced to log in with low graphic settigs.

Hint: when ubuntu is loading, the resolution starts OK, then goes bad right before the login screen is initialized. Does that mean that there's nothing wrong with my graphic cards?

What do you think? Cheers!