bug#39799: 28.0.50; Most emoji sequences don’t render correctly

[Eli Zaretskii <eliz@gnu.org>]

> From: Mike FABIAN <mfabian@redhat.com>
> Cc: rpluim@gmail.com,  39799@debbugs.gnu.org
> Date: Fri, 28 Feb 2020 22:10:43 +0100
> 
> Eli Zaretskii <eliz@gnu.org> さんはかきました:
> 
> >> From: Mike FABIAN <mfabian@redhat.com>
> >> Cc: Eli Zaretskii <eliz@gnu.org>,  39799@debbugs.gnu.org
> >> Date: Fri, 28 Feb 2020 18:30:12 +0100
> >> 
> >> U+24C2 is an Emoji which has both a text and an emoji presentation.
> >
> > I don't think I understand how this fact is relevant to which font
> > Emacs selects for a character.  Can you elaborate on the relation?
> 
> That means that U+24C2 should display using a black and white “text”
> font like Symbola and U+24C2 U+FE0E as well, if possible. But U+24C2
> U+FE0F should be displayed with a color emoji font like “Noto Color
> Emoji”, if possible.
> 
> That’s how it works in gedit for example.

If Gedit selects a font by looking at more than one codepoint (and I'm
not sure this is how it works in Gedit), then Emacs doesn't work that
way.

But regardless of how a font is selected, a single U+24C2 should be
displayed as its default glyph in a font, whereas U+24C2 followed by
U+FE0F should be displayed as a different glyph, the 16th variation of
U+24C2 provided by the font.  That variation could be a color one, or
it could be some other variation of the default glyph, it all depends
on what the font designer did.

> By browsing the emoji data files with Emacs, I noticed that all emoji
> which have text and emoji presentations *always* displayed in black and
> white for me (Symbola font) even when I tried to set the fontset to use
> “Noto Color Emoji” for these code points.

Emacs by default disregards the fontset for symbol and punctuation
characters.  Set use-default-font-for-symbols to nil to override that.

In any case, are these sequences displayed as composed characters?
Does "C-u C-x =" tell that the base character U+24C2 was composed with
the following variation selector?  According to the setup in
japanese.el, they should compose, if the font used for U+24C2 also
supports the variation selectors.