https://bugs.freedesktop.org/show_bug.cgi?id=103234
--- Comment #24 from Viktor Kecskes viktor.kecskes2@gmail.com --- (In reply to Marek Olšák from comment #23)
The xorg ati driver is for the radeon kernel module. The xorg amdgpu driver is for the amdgpu kernel module.
If the xorg driver is missing for the respective kernel module, the X server will use the generic xorg modesetting driver instead. The behavior might be the same because the modesetting driver has similar capabilities as the hw-specific drivers.
My mistake...I took an other look on the system, and the KDE scale method was set to smooth (The default is accurate). This is reproducable System > compositor > scale method > accurate = crash System > compositor > scale method > smooth or crisp = Can Alt + Tab without crash. So you are right, the different X11 driver doesn't solve the problem.