[BUG] @-mention session typeahead doesn't trigger for Korean (IME-composed) session names on native Windows

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When @-mentioning another Claude Code session by name in the prompt (cross-session messaging, typeahead added in v2.1.232), the suggestion list never appears when the target session's name is Korean.

Typing @ followed by Korean characters composed through the Windows Korean IME (e.g. to match a session renamed to "테스트") produces no session suggestions at all.

Renaming the exact same session to an ASCII name (e.g. "test") makes the typeahead work immediately — typing @tes shows the session in the suggestions. So the messaging feature itself works on native Windows; only the typeahead matching for IME-composed (CJK) input fails.

Suspected cause: IME composition (preedit) text may never reach the typeahead matcher, or only after composition commit which the matcher misses. Possibly related to the Windows Korean IME regression in #35307 (preedit/candidate window anchored to wrong position) and other CJK IME issues (#25186, #27857).

What Should Happen?

The @-mention typeahead should suggest sessions whose names match the typed Korean characters, the same way it does for ASCII names (and as it works on Linux).

Error Messages/Logs

Steps to Reproduce

1. On native Windows, start two Claude Code sessions in separate terminals.
2. In session A, run /rename 테스트 (any Korean name).
3. In session B, type @ then Korean characters, e.g. 테, using the Windows Korean IME.
4. Observe: no session suggestions appear.
5. In session A, run /rename test. In session B, type @tes.
6. Observe: the session now appears in the typeahead.

Claude Model

Other

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.241

Platform

Other

Operating System

Windows

Terminal/Shell

Other

Additional Information

• OS: Windows 11 Pro (build 10.0.26200), Korean IME (Microsoft IME).
• Terminal: Orca desktop app's built-in terminal (TERM_PROGRAM=Orca). The same Korean IME composition issues are reported across terminals in #35307 / #25186, so this is likely not terminal-specific, but I have not verified in Windows Terminal.
• Cross-session messaging itself (ListAgents/SendMessage, named-pipe delivery) works fine on this machine — only the @-mention typeahead fails for Korean input.