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

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: mfabian@redhat.com,  39799@debbugs.gnu.org
> Date: Fri, 28 Feb 2020 22:47:36 +0100
> 
> -fn "Noto Color Emoji" doesnʼt change the default font for me for some
>  reason, but if I change the font after startup then those sequences
>  display correctly.
> 
>   (char-table-range composition-function-table #xFE0F)
> => ([".." 1 compose-gstring-for-variation-glyph])

OK, so this feature already works for suitable fonts.

> and #xFE0F is always composable according to composite.c, so I donʼt
> understand why composing only works with Noto Color Emoji. Or does the
> font need specific support for it?

Yes, the font needs to have glyph variations, see
font-variation-glyphs and its underlying font-backend method
get_variation_glyphs.