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

From: Eli Zaretskii <[email-masked]>
Date: Sat, 29 Feb 2020 00:07:05 +0200

> From: Robert Pluim <[email-masked]>
> Cc: [email-masked],  [email-masked]
> 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.