Treat VS16 emoji-presentation sequences as double width

[Junyi-99]

Problem

A text-presentation base codepoint followed by U+FE0F (variation selector-16) requests emoji presentation, which modern terminals and unicode-width's string-level width() render as two columns — e.g. ❤️ (U+2764 U+FE0F), ⚠️ (U+26A0 U+FE0F).

vt100 sets a cell's wide flag from the base char alone (c.width() > 1 in Cell::set) and appends the VS16 as a zero-width combining char without re-checking, so the sequence is stored as a single narrow cell. Every column after the emoji is then shifted by one, which shows up as on-screen residue when the line is redrawn or scrolled.

Repro:

let mut p = vt100::Parser::new(1, 20, 0);
p.process("A\u{2764}\u{FE0F}B".as_bytes());
// before: cell(0,1) "❤️" is_wide() == false, cell(0,2) == "B"  (shifted)
// after:  cell(0,1) is_wide(), cell(0,2) is_wide_continuation(), cell(0,3) == "B"

Fix

In Screen::text's combining-character (width == 0) branch: after appending the combiner, if the cell's full contents now measure two columns by UnicodeWidthStr::width while the cell is still flagged narrow, promote it to wide and insert a wide-continuation cell. If that continuation column already held the first half of an existing wide glyph, clear that glyph's continuation too — mirroring the existing width > 1 placement path so no continuation is left orphaned.

Keying on the string-level width keeps the parser in agreement with unicode-width (and the terminals driving it).

Tests

tests/emoji_width.rs covers the core promotion and the overlapping-wide-glyph (orphan-continuation) case. Existing suite stays green.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Fixes a width-handling bug where a base character followed by U+FE0F (VS16) was stored as a single narrow cell, causing subsequent text to shift and leave on-screen residue. The fix detects when a combining character promotes a base to double-width emoji presentation and widens the cell, adding a wide continuation while cleaning up any glyph it clobbers.

Changes:

• In screen.rs, after appending a zero-width char to the base cell, detect when the resulting string width exceeds 1 and the cell is still narrow; widen it, add a wide-continuation cell, and clear any orphaned continuation from a pre-existing wide glyph.
• Expose Cell::set_wide as pub(crate) so Screen can promote a cell to wide.
• Add regression tests in tests/emoji_width.rs covering basic VS16 promotion and promotion over an existing wide character.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/screen.rs	Implements VS16 width promotion logic when a combining char widens the base cell.
src/cell.rs	Makes set_wide crate-visible so screen.rs can flag the promoted cell as wide.
tests/emoji_width.rs	Adds regression tests for VS16 emoji width handling, including the orphaned-continuation case.

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