Dear Maintainer,

Whenever I do a cold boot, I see the boot messages on my laptop screen and
monitor in parallel. Whenever GDM starts however, backlight is set to be off.
The secondary monitor still shows the gdm background. The login prompt however
remains on the primary laptop screen and thus cannot be interacted with.

Booting with no external monitors connected works fine.

Additionally, the backlight brightness cannot be adjusted when gdm is active
(but I think this is to be expected due to permissions?).

This on a mid 2013 macbook pro retina,
00:02.0 VGA compatible controller: Intel Corporation Device 0a2e (rev 09)
[    2.598451] fbcon: inteldrmfb (fb0) is primary device
[    3.996610] i915 0000:00:02.0: fb0: inteldrmfb frame buffer device
model name: Intel(R) Core(TM) i5-4258U CPU @ 2.40GHz

It does not matter whether the display is connected via hdmi or display-port.