Comment # 5 on bug 904139 from Frerk Meyer

Same here with:
Name: kdm
Version: 4.11.15-13.20
Arch: x86_64

After logout the screen is as dark as it can get. Hitting keys or using th
mouse or clicking with the mouse doesn't change the background brightness.
But as soon as I use the special hotkey on my Lenovo Thinkpad T400s for raising
the brightness it is restored to full brightnes instantly.