Embedders that render text outside the terminal grid need to predict
how many cells a codepoint will occupy once it is written to the
terminal. The immediate motivation is IME preedit overlay rendering:
measuring preedit text with font APIs (e.g. CoreText advances) can
disagree with the terminal's unicode table on ambiguous-width CJK and
emoji, causing the overlay to visibly jump when the composed text
commits and reflows through the real grid layout.
This exposes the exact width table the terminal print path already
uses, so overlays are column-accurate by construction. From C:
uint8_t w = ghostty_unicode_codepoint_width(0x4E00); // 2
And from the Zig module:
const vt = @import("ghostty-vt");
const w = vt.unicode.codepointWidth(0x4E00); // 2
The function is total over its input: 0 for zero-width codepoints
(controls, combining marks, default-ignorables, surrogates), 2 for
wide codepoints (East Asian Wide/Fullwidth, regional indicators,
clamped at 2), and 1 for everything else, including invalid values
beyond U+10FFFF.
Perf: uses the LUT lookup we use for the main core terminal
Binary size: the width table was already linked into libghostty-vt
via the print path, so this adds only the exported wrapper.
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