[BUG] IME composition underlines not displayed in Claude Code editor tab (webview) on 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?

IME composition underlines not displayed in Claude Code editor tab

What Should Happen?

When typing Japanese text in the Claude Code chat input field (opened as an editor tab, not terminal), IME composition underlines are not rendered. The solid underline (active conversion segment) and dotted underline (inactive segments) are both missing. This makes it impossible to visually identify which portion of text is selected for conversion, especially when adjusting the conversion range with Shift+Arrow keys.

Error Messages/Logs

N/A

Steps to Reproduce

1. Open Claude Code in Cursor as an editor tab (not terminal)
2. Switch to a Japanese IME
3. Type a Japanese sentence that requires conversion
4. Try adjusting the conversion segment with Shift+Arrow keys

Expected: Solid and dotted underlines appear beneath the text to indicate active/inactive conversion segments.

Actual: No underlines are displayed during composition.

Claude Model

Not sure / Multiple models

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.81 (Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Cursor

Additional Information

Environment

• OS: Windows 11
• Editor: Cursor (VSCode-based)
• Claude Code location: Editor tab (webview), not terminal
• IME: ATOK and Microsoft IME (both reproduce the issue)

Notes

• Cursor's own chat input (Composer) displays IME composition underlines correctly.
• Standard editor tabs in Cursor also display them correctly.
• This issue is specific to the Claude Code extension's webview input field.
• Unlike the many open terminal-based IME issues (#39644, #39245, #38520, etc.), this is a webview-specific problem.