Comment 7 for bug 1165965

Reports of further experimentation, using kernel 3.8.0-25-generic.

Xrandr can see the resolution of my external display, but it shows as disconnected. For instance:

 I can get the external display to partially work by using xrandr. For instance:
$ xrandr
Screen 0: minimum 320 x 200, current 3360 x 1080, maximum 32767 x 32767
eDP1 connected 1440x900+1920+0 (normal left inverted right x axis y axis) 286mm x 179mm
   1440x900 59.8*+
   1360x768 59.8 60.0
   1152x864 60.0
   1024x768 60.0
   800x600 60.3 56.2
   640x480 59.9
VGA1 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
DP1 disconnected 1920x1080+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
   1920x1080_60.00 60.0*
HDMI2 disconnected (normal left inverted right x axis y axis)
DP2 disconnected (normal left inverted right x axis y axis)

I can enable the external monitor output with:
$ xrandr --output DP1 --mode 1920x1080_60.00 --left-of eDP1

This works but xrandr shows the display as still disconnected (exactly the same output as above). Consequently, Unity gets quite confused and windows, title bars and menus don't work as expected. Nevertheless, the underlying display driver seems to be operating correctly.

This suggests that the problem is in whichever software layer determines that the external display is "connected".