bug#56691: outline-minor-mode displays navigation arrow buttons as emojis

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: Lars Ingebrigtsen <larsi@gnus.org>,  hello@paulwrankin.com,
>   56691@debbugs.gnu.org
> Date: Fri, 29 Jul 2022 14:48:01 +0200
> 
>     Eli>   (font-get (font-at 0 nil "😄") :family)
>     Eli>    => Symbola
> 
> There are dozens if not hundreds of fonts capable of displaying colour
> emoji.

Really, hundreds?  Are you sure?  Dozens don't scare me, and I thought
there are actually very few?

> Is it such a bad thing if we donʼt use the emoji when we could?

You mean, by default assume color Emoji aren't supported?  That'd
contradict what the documentation currently says, and is a strange
thing to do in such features.

>     Eli>   (defun display-color-emoji-p (&optional frame)
>     Eli>     "DOC"
>     Eli>     (and (display-multi-font-p frame)
>     Eli> 	 (let ((font-obj (font-at 0 (frame-selected-window frame) "😄")))
>     Eli> 	   (and (fontp font-at)
>     Eli> 		(member (font-get font-obj :family)
>     Eli> 			list-of-known-emoji-font-families)))))
> 
> That will return nil here, but in this particular terminal emulator,
> colour emoji display fine.

Terminal emulators is a separate issue.  If we need to support that,
we will have to let the user specify that.  (The current code doesn't
support that, either, right?)

> Iʼve not been following, but I assume thereʼs a "damn the torpedoes,
> I know what Iʼm doing" option for this stuff.

But the option's value is not a simple scalar, it is a list of
preferences, and they are supposed to be used in the order listed when
possible.  How do you reconcile that with "I know what I'm doing"?
Are users supposed to know in advance, for every display they use,
what are its capabilities?  And even if they do, are they supposed to
configure Emacs to use the lowest common denominator of those
displays?