Yes, I forgot to say: This is all about fullscreen frames.

You never use them. I use fullscreen exclusively, for Emacs.

When I am in Emacs (which is all day every day), I just want Emacs in
front of me. No menu bars, titlebars. or anything!

:)

Firefox does the stupid effect in full screen mode, too, turns out.
So we can't look there for the answer.

I don't think the correct solution is to change a setting on the OS,
since railwaycat Emacs does the right thing without external adjustments.

This bug is in fact what has kept me on his hacked Emacs all these
years. But due to other bugs I have been forced to upgrade to Emacs 30,
and I don't think he's going to update from 29.