X.Org colour corruption (1.20.4/Debian)

[Benjamin Ketcham]

Recent (<= 1 month) Debian install.  Tried both Xfce and LXDE, same
problem, seems to be an underlying X server issue.  Trying to play almost
any video (.mp4) causes all the screen colours to go "crazy".
Also, switching to a text console with ctrl-alt-F2, and then back (alt-F7)
scrambles the colours in the same way.
Logging out and back in resets the colours, but "xrefresh" does not.

This problem is new as of the new Debian install, did not happen with a
slightly older install (~1year); I'm not sure, but guessing, that X.Org has
changed in this time.
Looks very much like the kind of colormap problems we used to see with
8bpp/256-color pseudocolor displays; haven't seen it in many years and I
assumed the problem was "fixed" by 24/32bpp displays.  But no!
So far, can't find any useful discussion of this issue; sorry if it's a
FAQ, but it's not well-documented if so.
FWIW, I've done "apt-get update" and "upgrade".

--Benjamin

----
X.Org X Server 1.20.4
Linux bandit 4.19.0-10-amd64 #1 SMP Debian 4.19.132-1 (2020-07-24) x86_64
GNU/Linux
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