[BUG] Korean text causes background shading overflow after prompt submission

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 submitting prompts containing Korean (CJK) characters, the background shading extends beyond the actual text boundaries. The gray highlight "overflows" past where the text ends, and the overflow amount is proportional to the number of Korean characters.

This is a post-submission display rendering issue, not an IME input composition issue.
<img width="582" height="153" alt="Image" src="https://github.com/user-attachments/assets/f991c942-f650-4856-b04e-ff70c4ac4e7e" />

<img width="920" height="220" alt="Image" src="https://github.com/user-attachments/assets/dbd7cdca-ff09-4b56-a1b8-af7fde42dd84" />

What Should Happen?

Background shading should match the exact display width of the text. Lines containing Korean characters should render with the same precision as English-only lines.

Error Messages/Logs

No error messages - this is a visual rendering bug only.

Steps to Reproduce

1. Open Claude Code CLI in terminal
2. Type a prompt containing Korean text:

English text
한글 텍스트

1. Press Enter to submit
2. Observe: The line with Korean text ("한글 텍스트") has gray shading extending beyond the text

Note: The bug is visible on lines containing Korean characters. English-only lines display correctly.

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

2.0.73

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Xterm

Additional Information

• Claude Code Version: 2.0.73
• OS: macOS 15.6.1 (24G90)
• Architecture: arm64 (Apple Silicon)
• Terminal: Ghostty (xterm-ghostty)
• Shell: zsh
• Locale: en_US.UTF-8