[BUG] Korean IME: trailing character dropped to next line when pressing Enter

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?

Title
Korean IME: trailing character dropped to next line when pressing Enter

Body
## Environment

• OS: Windows 11 (Build 26200)
• Shell: bash (Git Bash)
• Input: Korean IME (한글 입력기)
• Claude Code: latest

## Steps to reproduce

1. Type a Korean string in the prompt input (e.g., 안녕하세요)
2. Press Enter immediately while the last syllable is still in IME composing state

## Expected
Full message is submitted: 안녕하세요

## Actual
The last syllable is left as a new line above the prompt and the message
submitted only contains the preceding characters: 안녕하세 + a stray 요
on the next line. The user has to retype/clean up before sending.

## Workaround
Press Space before Enter (forces IME to commit the composition). This
adds a trailing space but the message is sent intact.

## Impact
Affects all Korean users (and likely Japanese/Chinese IME users) on
Windows. It is a constant friction during typical use because Korean
input often ends mid-composition when the user reaches Enter.

## Suggested fix
Detect IME composition end before treating Enter as submit; or follow
VS Code / modern terminal behavior which commits the composition first
and then submits on the next Enter (so the first Enter just commits).

"저는 한국 사용자이며 매일 발생합니다"

What Should Happen?

Title
Korean IME: trailing character dropped to next line when pressing Enter

Body
## Environment

• OS: Windows 11 (Build 26200)
• Shell: bash (Git Bash)
• Input: Korean IME (한글 입력기)
• Claude Code: latest

## Steps to reproduce

1. Type a Korean string in the prompt input (e.g., 안녕하세요)
2. Press Enter immediately while the last syllable is still in IME composing state

## Expected
Full message is submitted: 안녕하세요

## Actual
The last syllable is left as a new line above the prompt and the message
submitted only contains the preceding characters: 안녕하세 + a stray 요
on the next line. The user has to retype/clean up before sending.

## Workaround
Press Space before Enter (forces IME to commit the composition). This
adds a trailing space but the message is sent intact.

## Impact
Affects all Korean users (and likely Japanese/Chinese IME users) on
Windows. It is a constant friction during typical use because Korean
input often ends mid-composition when the user reaches Enter.

## Suggested fix
Detect IME composition end before treating Enter as submit; or follow
VS Code / modern terminal behavior which commits the composition first
and then submits on the next Enter (so the first Enter just commits).

"저는 한국 사용자이며 매일 발생합니다"

Error Messages/Logs

Steps to Reproduce

Korean IME: trailing character dropped to next line when pressing Enter

Body
## Environment

• OS: Windows 11 (Build 26200)
• Shell: bash (Git Bash)
• Input: Korean IME (한글 입력기)
• Claude Code: latest

## Steps to reproduce

1. Type a Korean string in the prompt input (e.g., 안녕하세요)
2. Press Enter immediately while the last syllable is still in IME composing state

## Expected
Full message is submitted: 안녕하세요

## Actual
The last syllable is left as a new line above the prompt and the message
submitted only contains the preceding characters: 안녕하세 + a stray 요
on the next line. The user has to retype/clean up before sending.

## Workaround
Press Space before Enter (forces IME to commit the composition). This
adds a trailing space but the message is sent intact.

## Impact
Affects all Korean users (and likely Japanese/Chinese IME users) on
Windows. It is a constant friction during typical use because Korean
input often ends mid-composition when the user reaches Enter.

## Suggested fix
Detect IME composition end before treating Enter as submit; or follow
VS Code / modern terminal behavior which commits the composition first
and then submits on the next Enter (so the first Enter just commits).

"저는 한국 사용자이며 매일 발생합니다"

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

e.g.,1.0.123(Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

Korean IME: trailing character dropped to next line when pressing Enter

Body
## Environment

• OS: Windows 11 (Build 26200)
• Shell: bash (Git Bash)
• Input: Korean IME (한글 입력기)
• Claude Code: latest

## Steps to reproduce

1. Type a Korean string in the prompt input (e.g., 안녕하세요)
2. Press Enter immediately while the last syllable is still in IME composing state

## Expected
Full message is submitted: 안녕하세요

## Actual
The last syllable is left as a new line above the prompt and the message
submitted only contains the preceding characters: 안녕하세 + a stray 요
on the next line. The user has to retype/clean up before sending.

## Workaround
Press Space before Enter (forces IME to commit the composition). This
adds a trailing space but the message is sent intact.

## Impact
Affects all Korean users (and likely Japanese/Chinese IME users) on
Windows. It is a constant friction during typical use because Korean
input often ends mid-composition when the user reaches Enter.

## Suggested fix
Detect IME composition end before treating Enter as submit; or follow
VS Code / modern terminal behavior which commits the composition first
and then submits on the next Enter (so the first Enter just commits).

"저는 한국 사용자이며 매일 발생합니다"