[BUG] Korean IME: text after cursor disappears while composing mid-line, reappears on commit (macOS, iTerm2)

Environment

• Claude Code version: 2.1.204
• Platform: macOS 27.0 beta (Build 26A5378j), Apple Silicon (MacBook Air, Mac16,13)
• Terminal: iTerm2 3.6.11
• Input method: Apple built-in Korean 2-Set (두벌식)

Description

When editing in the middle of an existing line in the prompt input, starting Hangul (Korean) IME composition makes all text after the cursor visually disappear. The trailing text reappears as soon as the composition is committed (e.g. by pressing Space). No data is lost - the buffer is intact - but the line looks truncated during composition, which makes mid-line editing very disorienting.

Typing Korean also feels noticeably laggier than ASCII overall, which may share the same root cause.

Steps to reproduce

1. Run claude in iTerm2 with the macOS Korean 2-Set input source active.
2. Type a Korean sentence, e.g. 안녕하세요 반갑습니다.
3. Move the cursor back into the middle of the line with the Left arrow key.
4. Start typing a Hangul syllable so IME composition begins (e.g. ㅇ -> 아 -> 안).
5. Observe: all characters after the cursor disappear while the syllable is being composed.
6. Press Space (composition commits) -> the trailing text is re-rendered and reappears.

Expected behavior

Text after the cursor stays visible during IME composition, as it does when inserting ASCII mid-line.

Actual behavior

The line appears truncated at the cursor for the whole duration of the composition, and is only repainted after the composition commits.

Notes

• Looks like the same family as #5126 (closed but symptom persists in 2.1.204), #22732, #16322, #22853 - i.e. the TUI's own input-line rendering conflicting with the terminal's marked-text (preedit) overlay while the real terminal cursor is hidden.
• Unrelated to the paste mojibake fixed in 2.1.195 (that one no longer reproduces here).