See the screenshots posted by Erik at the beginning of this thread[1]
and mine[2] for comparison.

[1] https://debbugs.gnu.org/cgi/bugreport.cgi?bug=42184#5
[2] https://debbugs.gnu.org/cgi/bugreport.cgi?bug=42184#8

My aim was to allow you (Org maintainers) to commit this verbatim to the
org-mode repository, where you probably want to keep supporting Emacs
≤26.3.

NB: I'm far from an Elisp wizard, there may more elegant ways to solve
this than my backquotefest; suggestions welcome.

> Relatedly (but not crucial to solving this bug), should org-block and
> org-block-background also have :extend t?  It would make sense to me
> since they delimit visual "blocks".

I still think the answer to the above is "yes".  Will add to my patch if
you agree.

> Last question (that one might be answered somewhere in Org's
> documentation): how should we patch org-mode?  First on emacs-27, then
> on org-mode's maint branch?