[BUG] Multi-line input causes scrambled rendering in input area

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?

Description

When typing or editing multi-line input in the Claude Code input area, the rendering becomes visually scrambled. Dashed border lines bleed into the text area, and ghost/duplicate lines of text appear below the actual input. The actual input content is received correctly — the issue is purely visual.

Actual Behavior

• Dashed border lines bleed into the input text area
• Ghost/duplicate lines of previously typed text appear below the input
• Cursor positioning appears incorrect visually

Screenshot

<img width="950" height="440" alt="Image" src="https://github.com/user-attachments/assets/1d85b699-a1bf-4251-b8e5-18894c5dc24f" />

Environment

• Claude Code version: 2.1.34 (Claude Code)
• Terminal: iTerm2 (xterm-256color)
• Terminal size: 80x25
• OS: macOS 14 (Sonoma), Darwin 23.6.0
• Character encoding: UTF-8

What Should Happen?

Input text should render cleanly without visual artifacts.

Error Messages/Logs

Steps to Reproduce

1. Open Claude Code in an 80x25 terminal
2. Type a multi-line message in the input area (e.g., two or more lines)
3. Observe garbled rendering — border lines overlapping text, duplicate text appearing

Claude Model

Opus

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.34

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_