fix: correct terminal display width for Thai sara am and similar SpacingMark characters - #2

Summary

string-width v8 uses Intl.Segmenter and treats "ทำ" as a single grapheme cluster (width 1), but Thai sara am (ำ, U+0E33) occupies its own terminal column — correct width is 2. InputPrompt.tsx called stringWidth() directly on multi-character strings, causing ghost text wrapping and mouse click detection to be off by 1 column per sara am occurrence.

Details

Root cause: stringWidth("ทำ") === 1 (string-width v8, wrong) vs stringWidth("ท") + stringWidth("ำ") === 2 (per-code-point, correct). Unicode NFD decomposition is not a viable alternative: Thai sara am (U+0E33) has no canonical decomposition so NFD leaves it unchanged, and a naive NFD-then-per-code-point approach breaks emoji width (e.g. 👍🏿 → 4 instead of 2 by expanding skin-tone modifiers).

New getStringDisplayWidth(str) in textUtils.ts — hybrid strategy per grapheme cluster:

• Single code-point or emoji cluster (ZWJ sequences, skin-tone modifiers): delegate to getCachedStringWidth(cluster) — emoji remain correctly 2
• Multi-code-point non-emoji cluster (e.g. "ทำ"): sum per-code-point widths — each SpacingMark character contributes its own terminal column

// Before — wrong for Thai sara am
const usedWidth = stringWidth(textBeforeCursor); // "ทำ" → 1

// After — correct
const usedWidth = getStringDisplayWidth(textBeforeCursor); // "ทำ" → 2

InputPrompt.tsx: All stringWidth(str) calls replaced with getStringDisplayWidth(str); direct string-width import and getCachedStringWidth import both removed — InputPrompt.tsx now depends on only getStringDisplayWidth for all width calculations.

Related Issues

How to Validate

1. Type Thai text with sara am (e.g. ทำงาน) into the input prompt
2. Trigger ghost/autocomplete text containing ทำ — confirm it wraps at the correct column
3. Mouse-click on a line containing ทำ — confirm click lands on the correct character

npm test -w @google/gemini-cli -- src/ui/utils/textUtils.test.ts
npm test -w @google/gemini-cli -- src/ui/components/InputPrompt.test.tsx

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
    • npm run
    • npx
    • Docker
  • Linux
    • npm run
    • npx
    • Docker

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