Wait wait wait I've got it back.
If I turn on "Automatic graphics switching" off in the Energy Saver preference, I can reproduce the issue. Turning it on again resolves the issue the next time the window is on the DisplayLink screen.
Point is, we already tried playing with this setting before the SMC reset and it did not make any difference either way. Definitely an inconsistency about deciding which GPU or SW renderer is rendering which buffer.
So YMMV but it would be interesting to know if any affected system on the forum does NOT have a discrete GPU.
Will update Apple with the finding.
Kind regards,
Carlo
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