Every time I start a new plasma session and kwin launches, it will have
a series of graphical glitches which make the session unusable,
including:
- Not displaying the system taskbar.
- Not displaying any part of an application window which reaches the
bottom half of a screen.
- Having issues with graphical effects such as drawing rounded window
borders.
- Getting half of the screen in black for a while after issuing a
restart command.
(Among others)

When this happens and I restart the computer (a simple plasma session
restart does not suffice), next time it usually starts with compositing
disabled, which allows for kwin to start correctly, and compositing
will work fine once I reenable it in that same session, but then the
problem will happen again in the next restart.

The file ~/.xsession-errors will afterwards show many instances of this
line:
[2594:2594:0901/114421.740314:ERROR:shared_context_state.cc(73)] Skia
shader compilation error

(See also https://bugs.kde.org/show_bug.cgi?id=441871 for more details
and for the full xsession-errors)

I suspect there might be some issue between the current kwin version
and the video drivers (this is on an AMD Vega 56 card using the mesa
drivers, currently libglx-mesa0==21.2.1-2).

The issue is present whether the compositing is set to OpenGL 2.0,
OpenGL 3.1, or XRender.

Sounds like a broken mesa stack or broken GPU. Reading the upstream bug
also makes me wonder. I myself am running on amdgpu without any hitches.

Best

Norbert

Tried downgrading all the mesa packages to the ones in stable (as of
now that is mesa 20.3.5-1), with which kwin (from stable, version
5.20.5) was previously working correctly in this same setup, but the
problem persists.

The GPU still works as expected with a live CD image of some other
distro such as opensuse.

And this problem by the way goes away if I downgrade kwin to 5.20.5
from stable, so I doubt it's a mesa issue.