Comment # 3 on bug 103317 from Torsten Kessler

Created attachment 134912 [details]
Output of xrandr --verbose

As suggested I turned EnablePageFlip off:

Section "Device"
        Identifier "AMD R9 390"
        Driver "amdgpu"
        Option "DRI" "3"
        Option "EnablePageFlip" "off"
#       Option "TearFree" "true"
EndSection

But this doesn't avoid the problem. As before, everything works fine when I
switch to FHD or radeonsi.