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

[Eli Zaretskii <eliz@gnu.org>]

> Cc: gregory@heytings.org, 60841@debbugs.gnu.org, kevin.legouguec@gmail.com
> Date: Sun, 29 Jan 2023 21:33:38 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> 
> I think face-differs-from-default-p should look like this:
> 
> (defun face-differs-from-default-p (face &optional frame)
>   "Return non-nil if FACE displays differently from the default face.
> If the optional argument FRAME is given, report on face FACE in that frame.
> If FRAME is t, report on the defaults for face FACE (for new frames).
> If FRAME is omitted or nil, use the selected frame."
>   (let ((attrs
> 	 (delq :inherit (delq :extend (mapcar 'car face-attribute-name-alist))))
> 	(differs nil))
>     (while (and attrs (not differs))
>       (let* ((attr (pop attrs))
> 	     (attr-val (face-attribute face attr frame t)))
> 	(when (and
> 	       (not (eq attr-val 'unspecified))
> 	       (not (equal attr-val (face-attribute 'default attr frame)))
> 	       (display-supports-face-attributes-p (list attr attr-val)
> 						   frame))
> 	  (setq differs attr))))
>     differs))

Actually, I take this back: the 'equal' part is already done by
display-supports-face-attributes-p.

The problem with :stipple is that we don't allow nil as the value of
:stipple.  If you evaluate

  (setq list-matching-lines-prefix-face 'default)
  (face-differs-from-default-p list-matching-lines-prefix-face)

then look in *Messages*, you will see:

  Invalid face attribute :stipple nil

So we need to treat :stipple specially, or maybe fix merge_face_ref to
allow the nil value.