Comment # 3 on bug 107053 from Michel Dänzer

From the Xorg log file:

[   158.424] (WW) RADEON(0): flip queue failed: Device or resource busy
[   158.424] (WW) RADEON(0): Page flip failed: Device or resource busy
[   158.424] (EE) RADEON(0): present flip failed

This looks like a page flip is attempted while another one is still pending.
Unfortunately, handling that better turns out to be a surprisingly tricky
problem, but I think the best place would be xserver/present/present_scmd.c,
delaying the page flip until the previous one has completed.

Meanwhile, if you can try a GNOME on Wayland session, that might work better.