bug#60841: 30.0.50; kill-ring-save pauses despite region being highlighted

[Eli Zaretskii <eliz@gnu.org>]

> From: Kévin Le Gouguec <kevin.legouguec@gmail.com>
> Cc: gregory@heytings.org,  60841@debbugs.gnu.org
> Date: Wed, 18 Jan 2023 23:16:34 +0100
> 
> > * :extend nil for both: they display differently (region will not be
> >   extended, default will be),
> > * :extend t for both: they display the same,
> > * default has nil, region has t: they display the same,
> > * default has t, region has nil: they display differently.

The default face is always extended, so it should be treated as
implicitly having the :extend attribute set.

I think face-differs-from-default-p should be fixed to either ignore
the :extend attribute like it ignores :inherit (since it could be
argued that :extend doesn't really control how the face affects
characters on display), or it should treat the default face as having
the :extend attribute with the value t.

Can you see if any of these changes cause any trouble in our own use
of face-differs-from-default-p?  AFAICT, it will actually fix a subtle
problem in diff-mode.el: if diff-changed face doesn't define
non-default colors, it will be still taken as different from
'default', which I think is contrary to what diff-mode expects.

If we make the above change in face-differs-from-default-p, would
using it for the purpose of this issue fix the problem?

> (Hm, and against my better judgement I went ahead and compared
> gui_supports_face_attributes_p vs tty_supports_face_attributes_p, and I
> see that they handle :extend differently and *mashes C-c C-c with
> forehead before fingers can type another wall of text*)

TTY frames always extend the color, that's the reason for the
difference.  But does this affect my proposal above?