bug#57693: 29.0.50; Is there a more reliable version of `char-displayable-p'?

[Eli Zaretskii]

> From: Ihor Radchenko <yantar92@posteo.net>
> Cc: stefankangas@gmail.com, yantar92@gmail.com, 57693@debbugs.gnu.org
> Date: Sun, 19 Feb 2023 12:08:48 +0000
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> or within text-properties
> >
> > Only if really needed.  Adding a text property for this will
> > complicate the display code which handles this, so I'd rather we
> > didn't unless we have a _very_ good reason.  And I don't see such a
> > reason, since we are talking about characters that cannot be displayed
> > by any available font, something that is unlikely to be limited to
> > just a small region of text.
> 
> The replacement may only make sense in particular context. For example,
> mode line indicator for newlines can be ?: or ?\ or ?/. If we want to
> display a more intuitive Unicode char like ?⏎ or ?‸, it does not make
> sense to replace all the ?⏎ in buffer with ?:.

We could have a special glyphless-char-display table for mode-line and
header-line.  Buffer-local values don't apply well to those anyway,
because they display strings, not buffer text.