Panic in vendored vt100 when resizing strands a wide character at the row boundary #1267

Summary

The vendored vt100 fork (vendor/vt100, v0.16.2) can panic when:

1. A two-column character is rendered near the right edge.
2. The terminal is narrowed so its continuation cell is truncated.
3. The remaining wide-character cell is overwritten.

The resulting panic is:

thread 'tokio-rt-worker' panicked at vendor/vt100/src/screen.rs:896:22:
called `Option::unwrap()` on a `None` value

This was observed in production with Construct v0.17.6. The omni-codex adapter crashed mid-session after repeated resizing and wide-character output.

Root cause

A two-column glyph occupies:

• A cell for which is_wide() is true.
• A continuation cell at col + 1 for which is_wide_continuation() is true.

When the terminal is narrowed, Grid::set_size calls Row::resize. Shrinking a row truncates its cell vector, but—unlike Row::truncate—does not clear a wide character when its continuation cell falls outside the new row width.

This leaves an is_wide() cell stranded in the final column.

When Screen::text later overwrites that cell, its wide-character cleanup path accesses pos.col + 1. That position is now outside the row, so drawing_cell_mut returns None and the unwrap() at screen.rs:896 panics.

Minimal reproduction

let mut parser = vt100::Parser::new(1, 10, 0);

// Two-column glyph at zero-based columns 8 (wide) and 9 (continuation).
parser.process(b"\x1b[1;9H");
parser.process("世".as_bytes());

// Narrow to 9 columns. The continuation at column 9 is truncated,
// leaving the wide cell stranded in the new final column.
parser.screen_mut().set_size(1, 9);

// Overwriting the stranded wide cell panics at screen.rs:896.
parser.process(b"\x1b[1;9HX");

Expected behavior

Resizing and subsequently overwriting the cell should not panic. If shrinking removes a wide character's continuation cell, the remaining half should be cleared.

Actual behavior

The overwrite panics at vendor/vt100/src/screen.rs:896 because the expected continuation cell no longer exists.

Suggested fix

When Row::resize shrinks a row, clear any wide-character cell stranded in the new final column. This should mirror the boundary cleanup already performed by Row::truncate.

A regression test covering the sequence above would also help prevent recurrence.

Environment

• Construct v0.17.6
• Vendored vt100 fork v0.16.2 (vendor/vt100)