I normally run KDE with the compositor disabled.  In this mode, all KDE
applications exhibit severe screen corruption: all new text or graphics that
appear have a garbled appearance, to the extent that text is completely
illegible.  The corruption sometimes disappears after a few seconds or after
the window is scrolled.  The problem does not affect GTK+ applications.  The
problem can be worked around by using systemsettings5 to tell KDE to enable the
compositor at startup.  However, simply changing the rendering backend (XRender
vs. OpenGL) has no effect.

I am using an onboard Intel HD Graphics 630 controller with the i915 driver. 
I'm not sure if it's the kernel driver/firmware which is causing the problem,
but based on past experience with Intel graphics glitches, I am tentatively
assuming it is.

The problem occurred after updating Tumbleweed from version 20210810 to version
20210827.  (In case it's relevant, this included an update of kernel-default
5.13.8 to 5.13.12, and from kernel-firmware-i915 from 20210719 to 20210812.) 
The problem persists in version 20210830.

Booting from the old 5.13.8 kernel doesn't work around the problem.  I could
also try reverting kernel-firmware-i915 if I can find or build an old package.