Comment 2 for bug 193419

Same for me : no way to get the external screen on. Dual screen never had work for me (since edgy).

When settings display on, it doesn't change anything.
$ gnome-display-properties
Resource Timestamp: 869674
Resource Configuration Timestamp: 869674
CRTC 49 Timestamp: 869674
CRTC 4a Timestamp: 869674
Output 4b Timestamp: 869674
Output 4c Timestamp: 869674
Output 4d Timestamp: 869674
New configuration
  output VGA off: 0 0 0 0
  output LVDS on: 0 0 1280 800
  output TV off: -1 -1 -1 -1
sorting
  output: -1 -1 -1 -1
  output: 0 0 0 0
  output: 0 0 1280 800
Sending client message
Resource Timestamp: 869674
Resource Configuration Timestamp: 869674
CRTC 49 Timestamp: 869674
CRTC 4a Timestamp: 869674
Output 4b Timestamp: 869674
Output 4c Timestamp: 869674
Output 4d Timestamp: 869674
New configuration
  output VGA on: 0 0 1280 1024
  output LVDS on: 0 0 1280 800
  output TV off: -1 -1 -1 -1
sorting
  output: -1 -1 -1 -1
  output: 0 0 1280 1024
  output: 0 0 1280 800
Close

$ xrandr
Screen 0: minimum 320 x 200, current 1280 x 1024, maximum 1280 x 1280
VGA connected 1280x1024+0+0 (normal left inverted right x axis y axis) 359mm x 287mm
   1280x1024 60.0*+ 75.0 59.9
   1024x768 75.1 60.0
   832x624 74.6
   800x600 75.0 60.3
   640x480 75.0 60.0
   720x400 70.1
LVDS connected 1280x800+0+0 (normal left inverted right x axis y axis) 261mm x 163mm
   1280x800 60.0*+ 60.0
   1280x768 60.0
   1024x768 60.0
   800x600 60.3
   640x480 59.9
TV disconnected (normal left inverted right x axis y axis)

Interesting information :
$ xrandr --output VGA --auto --left-of LVDS
xrandr: screen cannot be larger than 1280x1280 (desired size 2560x1024)

When I plug the external display and restart gdm, gdm is display in internal screen but at the size of the external screen !