>Thanks.
>
>Let's install this on master.  We can consider backporting to the
>emacs-28 branch after Emacs 28.1 is released.

Installed in commit a6abd06c73.

Sorry I forgot to respond to your question earlier ("if this is a 
regression, what was the last Emacs version where the reproduction 
recipe worked correctly?"), Eli.  I didn't know the answer, and 
didn't have time to find out then, but I should have said so 
explicitly.  (There was enough other information in my reply that 
I didn't want to delay sending it.)

As I promised, there are two follow-on changes.  I think it would 
make sense to also put them on 'master', but I'll attach them here 
so you can see for yourself; let me know what you think.

Best regards,
-Karl