https://bugs.debian.org/762676 means that, when docked, my laptop
boots to a state in which the greeter is not visible: the laptop's
backlight is off, and the external monitor is blank (grey background)
because the laptop was selected as the primary display.

Pressing the "backlight up" key (Fn+Home on my Thinkpad X220) brings
the brightness up by 1 step, but does not go beyond that. Additionally,
I get lots of these messages in the journal:

This might be because Debian-gdm has /bin/false as its shell:
brightness); Ctrl+Alt+F7 (switch back to gdm, backlight stays on).

Had the same problem.
Asus zenbook UX32VD, Intel HD Graphics 4000

I'm on debian stable now.
gdm3 is 3.14.1-7

Take a look https://bugs.freedesktop.org/show_bug.cgi?id=87286

It provided an idea to try more recent version of the package.
I upgraded "xserver-xorg-video-intel" from stable 2:2.21.15-2+b2 to
testing 2:99.917-2 and it has resolved the issue.

I used aptitude with, as I recall, 3rd suggestion to resolve dependencies.


I had another issue on many other and very recent distros and still have it:
decreasing (only) brightness press key work only once, next presses get
an unknown delay (30-60 seconds) to be finished.
increasing brightness works well when pressing it many times.
True for gnom3 and kde (4, 5).