Thai text broken in TUI: input editing + output rendering

Description

Claude Code TUI mishandles Thai combining characters (above/below vowels and tone marks, U+0E30–U+0E4E). These are zero-width marks that stack on the preceding consonant, but Claude Code counts them as 1 cell. This breaks both the input prompt and the rendered output.

Symptoms

Input box (worst impact — blocks composing Thai messages):

• Cursor block drifts off the actual caret position as you type
• Backspace deletes the wrong character
• Cursor visually overlaps glyphs (see screenshot)
• Selecting/editing Thai text is essentially unusable

Output rendering:

• Tool results and Claude responses containing Thai wrap mid-syllable
• Combining marks render shifted one cell to the right of their base

Reproduction

1. Run claude in any terminal (reproduced in macOS Terminal.app, Darwin 24.6.0)
2. Type Thai in the input, e.g. ฉันต้องการให้มันไม่บัคแบบนี้
3. Try to move the cursor or backspace — position is wrong

Environment

• Claude Code 2.1.145
• macOS Darwin 24.6.0

Likely the string-width logic isn't treating Unicode Mn (Nonspacing Mark) as zero-width. Same class of bug affects Lao, Khmer, Devanagari, Arabic combining marks.