Created attachment 137106 [details]
Working under mesa-common-dev 13.0.6-1+b2, plasma-workspace 4:5.8.6-2.1, Xorg 1.19.2 and kernel 4.15.0-rc9+ I do get weird screen distortions under KDE as soon as I enable my external 4K display. As KDE runs stable and fine with my integrated display in a much lower resolution I would guess this to be a memory allocation issue of nouveau. Other less memory intense desktop environments like Xfce still run fine under 4K. The issue is known since long (see also the KDE bug: https://bugs.kde.org/show_bug.cgi?id=368275)
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