Summary

Mouse-drag copy selection drifted whenever the selection crossed full-width characters (CJK, emoji). Mouse coordinates are terminal cell columns, but the copy path sliced rendered lines with String.prototype.slice, which counts UTF-16 code units. ASCII stays aligned (1 cell = 1 code unit), but each full-width character is 1 code unit and 2 cells, so every one before a selection endpoint shifted the copied text by one position.

• add cellRangeToCharRange in src/ui/lib/text.ts to convert an inclusive terminal-cell range into code-unit slice bounds, walking grapheme clusters so wide characters and emoji are never split
• convert cell columns to string indices before every slice in renderCopySelectionText (code-only and decorated paths, drag start and end) and in expandSelectionPoint (double/triple-click word and line selection)
• fix the file-header copy path too: renderFileHeaderCopyText now measures the gap and clamps by cells, matching DiffFileHeaderRow's cell-based layout for wide-character filenames
• keep zero-width characters at a selection boundary so invisible characters round-trip through the clipboard
• add a patch changeset

Repro

Drag-select the prefix export const message = 'こんにちは on this line:

export const message = 'こんにちは'; // greeting

That prefix is 29 UTF-16 code units but 34 terminal cells (five full-width characters, +1 cell each), so slicing by code units overshoots by five cells:

Testing

• bun run typecheck
• bun run lint
• bun run format:check
• bun run test:theme-contrast
• bun test
• bun run test:integration
• bun run test:tty-smoke