bug#57072: [BUG] update-glyphless-char-display and variation selectors

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: Axel Svensson <svenssonaxel@gmail.com>,  57072@debbugs.gnu.org
> Date: Tue, 16 Aug 2022 13:55:52 +0200
> 
> >>>>> On Wed, 10 Aug 2022 16:10:46 +0300, Eli Zaretskii <eliz@gnu.org> said:
> 
>     >> From: Axel Svensson <svenssonaxel@gmail.com>
>     >> Date: Tue, 9 Aug 2022 22:33:40 +0200
>     >> Cc: 57072@debbugs.gnu.org
>     >> 
>     >> > Now, VS16 is almost always composed with preceding characters, so I
>     >> > think you can only see it as acronym if you deliberately force Emacs
>     >> > not to compose it, e.g. by preceding it with U+20DD COMBINING
>     >> > ENCLOSING CIRCLE, or precede it and follow it by U+200C ZERO WIDTH
>     >> > NON-JOINER, or disable auto-composition-mode.
>     >> 
>     >> - Preceding it with U+20DD still produces the empty box
>     >> - Preceding it and following it by U+200C still produces the empty box
>     >> - Disabling auto-composition-mode produces the "VS16" acronym.
> 
>     Eli> Yes, I think this is because of the special composition rules we have
>     Eli> for VS16 (which are required to display Emoji sequences correctly).
> 
> I guess we could adjust the composition rules for U+FE0F, but getting
> that right could be tricky (there are many of them, and there will be
> ordering dependencies).

We could, but I'm not sure it's worth the hassle.  There's no
particular reason for people to want to display VS-16 as an acronym,
of all the ways, since it almost always should be composed.