On the Asus X570VD laptop with AMD Ryzen 7 with Radeon Vega Mobile Gfx, if you
use the brightness keys to reduce the screen brightness to the minimum level
under GNOME, the display flickers disturbingly.

This is reproducible under Ubuntu 18.04 with all updates applied (Linux 4.15).
It's also reproducible with AMDGPU-PRO All-Open drivers installed on top.
It's also reproducible under Linux 4.18.5.

It's hard to capture on camera, but you can get some appreciation of the issue
in this video:
https://youtu.be/tCoA45TJlAk

When the screen is in this state, the /sys/class/backlight/brightness file has
value 1. If I manually adjust to value 2, the flicker goes away.