Chinese IME input: characters overwritten/dropped when editing mid-text

Bug Description

When typing Chinese using an IME in the Claude Code CLI, characters are swallowed or overwritten if you move the cursor back into already-typed text and insert more characters there.

Steps to Reproduce

1. Open Claude Code CLI (claude)
2. Type some Chinese via IME, e.g. 你好世界
3. Use arrow keys to move cursor back between existing characters
4. Type more Chinese characters via IME

Expected: New characters inserted at cursor without affecting surrounding text.

Actual: Adjacent characters get overwritten or dropped.

Root Cause (likely)

CJK characters are 3 bytes in UTF-8 but occupy 2 display columns. Node.js readline miscalculates cursor offsets for wide characters, causing incorrect overwrites during mid-text insertion.

Environment

• OS: macOS 26.2 (Build 25C56)
• Claude Code: 2.1.159
• IME: macOS built-in Pinyin