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

[Robert Pluim]

>>>>> 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).
️
Robert
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