I am affected by the BUG: https://bugs.launchpad.net/ubuntu/+source/unity-greeter/+bug/874241 Otherwise, if like me you have a laptop connected to a second monitor of higher resolution, LIGHTDM at the login stage, mirrors the displays in both screens and assign to them a common resolution (1024X768) in my case, instead of extending the desktop (Primary screen with the greeter and secondary with just a logo as mentioned in the Multiple Monitors UX specifications book for 12.04).

Here is my xrandr -q

@L502X:~$ xrandr -q
Screen 0: minimum 320 x 200, current 1920 x 1848, maximum 8192 x 8192
LVDS1 connected 1366x768+309+1080 (normal left inverted right x axis y axis) 344mm x 193mm
   1366x768       60.0*+
   1360x768       59.8     60.0  
   1024x768       60.0  
   800x600        60.3     56.2  
   640x480        59.9  
VGA1 disconnected (normal left inverted right x axis y axis)
HDMI1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 510mm x 287mm
   1920x1080      60.0*+
   1600x1200      60.0  
   1680x1050      60.0  
   1280x1024      60.0  
   1440x900       59.9  
   1280x960       60.0  
   1280x800       59.8  
   1024x768       60.0  
   800x600        60.3     56.2  
   640x480        60.0  
DP1 disconnected (normal left inverted right x axis y axis)

I tried to force lightdm to execute some xrandr commands in order to set the right resolution for each monitor and extend the desktop, but I get a LOW GRAPHICS MODE ERROR (You're running in low graphics mode, your screen, input devices...did not get detected..)

I created a simple script named lightdmxrand.sh:

#!/bin/sh
xrandr --output HDMI1 --primary --mode 1920x1080 --output LVDS1 --mode 1366x768 --below HDMI1

And told lightdm to run it : /etc/lightdm/lightdm.conf

[SeatDefaults]
greeter-session=unity-greeter
user-session=ubuntu
greeter-setup-script=/usr/bin/numlockx on
display-setup-script=/home/hanynowsky/lightdmxrandr.sh

Someone knows what is wrong!? Thanks in advance.