[vim/vim] libvterm: characters can get a different width in a terminal window (PR #21213)

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h_east

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Sep 3, 2026, 12:35:53 PM (4 days ago) Sep 3

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Problem:  Some characters get a different width in a terminal window
          than in an ordinary window.
Solution: When the application provides a width function, use it as the
          only source for the width of a character.

libvterm looks up its own "fullwidth" table before calling the wcwidth() function that Vim provides. That table is from Unicode 9, while Vim follows a newer version, so U+1F93B and U+1F946 are two cells wide in a terminal window and one cell wide everywhere else. The tables also disagree about U+302A-302D and U+3099-309A, which are combining characters that the table reports as two cells wide.

Skipping the table when a wcwidth() function is provided makes that function the only authority, so a terminal cell gets the same width as the character gets in a buffer. A build of libvterm without such a function keeps using the table.

You can view, comment on, or merge this pull request online at:

https://github.com/vim/vim/pull/21213

Commit Summary

b942318 libvterm: characters can get a different width in a terminal window

File Changes

(2 files)

M src/libvterm/src/unicode.c (7)
M src/testdir/test_terminal.vim (33)

Patch Links:

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h_east

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Sep 3, 2026, 12:36:36 PM (4 days ago) Sep 3

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h-east left a comment (vim/vim#21213)

@leonerd Heads-up in case this is useful upstream: src/fullwidth.inc is
stale. Re-running find-wide-chars.pl under Perl 5.38 (Unicode 15.0) splits
{ 0x1f90d, 0x1f971 } into three ranges, because U+1F93B and U+1F946 carry
East_Asian_Width=Neutral in current Unicode data while the committed table
still reports them as two cells wide. Separately, the table covers
U+302A-302D and U+3099-309A, which are Mn, so vterm_unicode_width() returns
2 for them before mk_wcwidth() gets to its combining check.