bug#39360: Add :extend attribute to show-paren-mode expression face

[Eli Zaretskii]

> Cc: 39360@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Fri, 7 Feb 2020 03:27:15 +0300
> 
> > We left the region as it was to avoid too strong outcry.  In general,
> > region should also not extend, IMO.
> 
> How about the reasons of consistency?

It takes a back seat in this case.  (And in general, consistency is a
very weak argument in this case, because each face is used in
different circumstances.)

> I'd be fine if region was not extended. But since we do extend it, why 
> not make other face defaults in similar situations consistent with that?

Because, as explained previously, the fact that region extends is a
kind-of "retreat".  Doing that for many other faces would mean the
feature itself is useless, and probably should not have been coded in
the first place.  I don't think this is the case.