bug#37774: 27.0.50; new :extend attribute broke visuals of all themes and other packages

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 37774@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Wed, 23 Oct 2019 15:47:23 +0300
> 
> > I don't know, as I don't have a clear idea what faces there are
> > affected and why.  I hoped someone, preferably the Magit developers,
> > would describe that in enough detail to understand the situation.
> 
> The list of faces has been posted here already:
> 
> https://debbugs.gnu.org/cgi/bugreport.cgi?bug=37774#233

AFAIU, that's a list of faces one particular user decided to customize
to have them extended.  It's a far cry from the list of faces that
actually need to be extended, lest some important functionality will
suffer.  IOW, we need some rationale for each face, so that we could
consider that and decide whether or not to extend each one by default.

Besides, some of those in the list were already changed.

If too many faces in unbundled packages indeed need to change in that
way, we should consider additional measures.  That's why we need good
reasons for extending each face, not just "because they were before"
or because people were used to see them extended.

> As apparent from their names, most of them are used in a Diff output 
> buffer, similar to our diff-mode faces.

Most, but not all.  And I'm not yet convinced that every face with
"diff" in it must indeed be extended, we need to see examples of their
display, and we need to talk about that.

> In anticipation of your next question: no, they don't inherit from any 
> of the diff-mode faces.

I know.