Comment 3 for bug 779921

Actually external monitor with Unity is also OK in 832x624 but not in 1024x768 and 1280x1024.
see xrandr information below:

Screen 0: minimum 320 x 200, current 832 x 624, maximum 4096 x 4096
LVDS1 connected (normal left inverted right x axis y axis)
   1024x600 60.0 +
   800x600 60.3 56.2
   640x480 59.9
VGA1 connected 832x624+0+0 (normal left inverted right x axis y axis) 338mm x 270mm
   1280x1024 60.0 + 75.0
   1024x768 75.1 70.1 60.0
   832x624 74.6*
   800x600 72.2 75.0 60.3 56.2
   640x480 72.8 75.0 66.7 60.0
   720x400 70.1