use-default-font-for-symbols vs. emojis (was: Wrestling with set-fontset-font)

[Kévin Le Gouguec]

> From: Eli Zaretskii <eliz@gnu.org>
> Emacs by default prefers using the default font for displaying symbol
> and punctuation characters, if that font supports them.  Set
> use-default-font-for-symbols to nil to force Emacs always select the
> font according to your fontsets, when it needs to display such
> characters, even if the default font can display them.

Thank you!  (Note to self: when Info and Custom fail, there's always C-h
v *keyword TAB…)

*skims comment in fontset.c:face_for_char*

How interesting!

>          However, these considerations are unlikely to be relevant to
>          punctuation and other symbols, since the latter generally
>          aren't specific to any culture, and don't require
>          sophisticated OTF features.  */

Do emojis challenge this heuristic, since

- they can typically use "sophisticated OTF features" such as color,
- combining sequences[1] are a thing?

If so, could/should there be a way to distinguish "emoji" characters
from the more general "symbol" script, using e.g. data from
<https://unicode.org/Public/emoji/12.1/>?

(I hope at least /some/ of the above makes sense.  My knowledge of
Fonts, the Unicode and Everything is anecdotal at best…)


PS: Sorry for not replying to your message Eli; I'm not subscribed to
    help-gnu-emacs, and I can't find an NNTP feed for it…


[1] E.g. 🧟‍♂️, 🧟‍♀️, 🐕‍🦺, 🏴‍☠️, from
    <https://unicode.org/Public/emoji/12.1/emoji-zwj-sequences.txt>, or
    flag sequences from
    <https://unicode.org/Public/emoji/12.1/emoji-sequences.txt>; none of
    which work inside Emacs yet AFAICT.

    To be more precise: as of commit 9ee5af31, these examples appear
    "decombined".  I have no idea what controls this (Harfbuzz? Cairo?)…

    FWIW, on Debian Buster, these combinations work for some of XFCE's
    applications (e.g. Thunar, Mousepad) but not others (e.g. Terminal).
    (Well, some of these combinations anyway.  No application seems to
    give the faithful U+1F415 U+200D U+1F9BA the love it deserves…)