Serious artifacts on gtk3 applications in a headless, accelerated X on AMD

[marty1885]

If you'd like to put out an incentive for fixing this bug, you can do so at https://issuehunt.io/r/LibVNC/x11vnc

Describe the bug

GTK app is full of artifacts. But Qt applications seems to be fine; besides the title bar, that is provided by GTK. Curiously the main desktop seems to be fine. And this doesn't happen is I'm using X with the dummy video driver and llvmpipe.

To Reproduce

• Install mesa, amdgpu, X, x11vnc, sddm, etc...
• Setup virtual display using driver parameters
  • echo "options amdgpu virtual_display=0001:01:00.0,1" > /etc/modprobe.d/amdgpu.conf
  • 0001:01:00.0 is the bus ID, 1 is the number of virtual screens
• reboot
• start sddm
• connect to sddm using x11vnc -display :0 -auth $(find /var/run/sddm/ -type f)
• Login to Gnome (or xfce, xfce is even worse)
• Start any GTK application

Expected Behavior Clear image without artifacts.

Screenshots

Gnome:

XFCE:

Desktop (please complete the following information):

• OS and version: Arch Linux, aarch64
• GPU: RX 570
• Driver: Mesa 20.2 / Kernel 5.4
• Xorg version used: 1.20.9

[bk138]

You might want to play around with the various cmdline switches documented in the man page.