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

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Cc: Eli Zaretskii <eliz@gnu.org>,  Clément Pit-Claudel
>  <cpitclaudel@gmail.com>,  39360@debbugs.gnu.org
> Date: Fri, 07 Feb 2020 02:40:17 +0200
> 
> There are not too many faces that could be extended for the same
> reason why the region face is extended.  From all faces that I used
> only a few required customizing to add :extend t, namely:
> 
> mm-uu-extract
> org-block
> org-block-begin-line

These are up to the developers of the corresponding packages.
Although IMNSHO, the last one doesn't sound to me like a face that
should be extended by default.

> xref-file-header (because I customized it to have a grey background color
>                   like in diff-mode)

If you customize that face, you could also make it extend while at
that, no?  IOW, what you say doesn't sound like an argument for making
it extend by default, since by default it specifies only the
foreground color and the weight.