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

[Lars Ingebrigtsen <larsi@gnus.org>]

Eli Zaretskii <eliz@gnu.org> writes:

> I think we can currently only do that by looking at the details of the
> build.  Currently, color Emoji are supported only by builds with Cairo
> and HarfBuzz on Unix and by macOS builds.

We don't support colour emojis on Windows?  I didn't know that.

> But even that could be not enough, since the necessary font may not be
> installed.  So here's another idea: use font-at to check that it
> returns one of the known color Emoji fonts for an Emoji character.
> For example:
>
>   (font-get (font-at 0 nil "😄") :family)
>    => Symbola

Ah, right.  I didn't know that Symbola had coverage for these
characters.  Perhaps a really simple fix would be to just choose a
character that we're pretty sure will never be supported by non-Emoji
fonts?  Let's see...

Symbola doesn't have support for LARGE ORANGE CIRCLE "🟠", so we could
try that instead.  I know, it's brittle, but so is listing all known
colour emoji fonts.

The real fix would be to look at the actual glyph data produced, and see
whether it's colourful or not, wouldn't it?  Is that possible?  Or
alternatively -- query the font whether it has colour glyphs, but I
don't know whether that's easy, either.  Anybody know?