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: Sun, 22 Jan 2023 23:45:23 +0100
> 
> > 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.
> 
> I have been slowly converging toward "ignore :extend" being TRT.

Fine by me.

> * that seq-difference call makes me inexplicably nervous; I thought I
>   vaguely remembered debates on whether preloaded libraries {c,sh}ould
>   use seq.el functions, but then I see that "emacs-lisp/seq" is already
>   in preloaded-file-list, and e.g. rmc.el calls some of its functions.
>   Am I misremembering?

seq.el is indeed preloaded, so that ship has sailed.  But you still
need to make sure seq is loaded _before_ any preloaded file which uses
it, and in this case faces is loaded before seq, so you cannot use
seq-difference.

And, btw, I cannot say I understand why using seq-difference here is
justified.  Why not just call delq twice and be done?

> >> (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.
> 
> (Not sure I get your meaning here; on the Linux TTY I have on hand,
> (set-face-extend 'region nil) does disable color extension)

I'm sorry, you will have to look up the discussion that led to the
development of the :extend attribute; I cannot afford searching for
it.  The differences between TTY and GUI frames were one of the main
reasons why we introduced this attribute.  Maybe what I remember
happens only on some terminals.  Or maybe I'm misremembering and it
was because of underline and not the color.  But there is definitely a
difference.

> +(defun region-highlighted-p ()
> +  "Say whether the region is visibly highlighted.

Please drop the "Say" part, it's not our style.

And I'd use some other word instead of "highlighted", because that
could be misinterpreted (the region is supposed to be highlighted).
Something like "stands-out" perhaps?