Japanese IME: text duplicates/corrupts when inserting in the middle of input

Description

When using Japanese IME (macOS), if you type text in the input field, delete part of it, and then insert new text in the middle of the existing text, the input becomes corrupted — text gets duplicated or garbled.

Steps to reproduce

1. Type a Japanese sentence in the Claude Code input field
2. Move cursor to the middle of the text
3. Start typing new Japanese text (via IME) at the cursor position
4. The newly typed text duplicates or merges with existing text incorrectly

Expected behavior

Text should be inserted at the cursor position without corruption.

Actual behavior

Text gets duplicated or garbled. See screenshots below.

Screenshots

Screenshot 1: Text corruption after inserting in the middle
!screenshot1

Screenshot 2: Another instance of the same bug
!screenshot2

(Screenshots are from local temp files — attaching description instead)

• Screenshot 1: Input shows 決済一覧にライフティします お願いします — garbled after mid-insertion
• Screenshot 2: Input shows スクリプトルはないよ。会社名だもん。同じアイパス記録案件と同じアイパス — text duplicated ("同じアイパス" appears twice) after mid-insertion

Environment

• macOS (Darwin 23.6.0)
• Japanese IME (standard macOS)
• Claude Code CLI (latest)