Comment # 14 on bug 99784 from Valeriu Balaban

Created attachment 131429 [details]
Output of xrandr and dmesg

I have the same issue on Ubuntu 16.04 LTS with Mate desktop that runs on an
iMac 11. The second monitor is connected through a Display Port to VGA adapter
and the kernel selects it to be primary by default. 

After a fresh boot, since the second monitor is the primary one it contains the
login screen, however the screen is black. So I have to unplug it to make the
internal monitor (eDP) primary. If I plug again the external one, the internal
monitor turns black and the external becomes active and starts to display it's
content.

If I take a screenshot, I can see what is displayed on both monitors.