Comment 7 for bug 861426

Yes, confirmed this works, so your diagnosis looks correct :)

I did the following:

1) Set up a spanned laptop + external display configuration. This works, correct?

Correct this works fine and the main display works at my full resolution 1900x1200

2) Open up a terminal, and run "xrandr --output LVDS1 --off". This should turn off your laptop panel, leaving only the external display active. It should *also* not break your external display.

Correct, this turns off the laptop display, moves all windows onto the main display and the main display stays at full resolution with no problems.

3) Report whether or not (2) turned off your internal display without breaking your external display :)

Yes!