Re: buffer-face-set changes the fringe, is it a bug?

[Gregory Heytings via "Emacs development discussions." <emacs-devel@gnu.org>]

>
> > > It is not recommended to omit the @var{face}, as an omitted 
> > > @var{face} leads to unexpected behavior.
>
> Why not make that an error instead?
>

Another possibility, which I didn't think about earlier, would be to 
(implicitly) use an empty face, that is, to make omitting the face 
equivalent to specifying an empty face in 
xdisp.c:handle_single_display_spec().  This works perfectly well:

(defface empty-face '((t nil)) "")
(set-face-attribute 'fringe nil :background "red" :foreground "yellow")
(overlay-put (make-overlay (point) (point)) 'before-string (propertize "_" 'display `(left-fringe question-mark empty-face)))
(text-scale-adjust -1)
(text-scale-adjust +1)

Gregory