[BUG] Cursor renders on line below > prompt (regression since v2.1.73)

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

The input cursor renders on the line below the > prompt instead of on the same line. This creates a visual gap between the prompt character and where text is actually typed.

## Environment

• Claude Code version: 2.1.90 (and all versions after 2.1.73)
• Last working version: 2.1.73
• Terminal: tmux-256color inside tmux + iterm2
• OS: Linux 6.16.1 (devserver)
• Shell: zsh
• COLUMNS=0, LINES=0 reported inside Claude Code subprocesses
• stty size fails inside Claude Code subprocesses
• tmux pane dimensions: 136x34 (correctly reported by tmux)

## Notes

• Happens with and without a custom statusLine configured
• Happens with and without emoji characters in output
• Intermittent but very frequent — occurs multiple times per session
• Did NOT happen on v2.1.73

## Screenshots

<img width="391" height="95" alt="Image" src="https://github.com/user-attachments/assets/b2da8211-d5f6-41ec-90a7-2c1c5d6a0fd3" />

<img width="1258" height="332" alt="Image" src="https://github.com/user-attachments/assets/1902f1c2-5083-4d68-8fed-050331bfb7f3" />

What Should Happen?

Rendering glitch shouldn't happen

Error Messages/Logs

Steps to Reproduce

## Repro

1. Run Claude Code v2.1.74+ in tmux + iterm2
2. Observe the > prompt — the blinking cursor appears on the next line below it

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.73

Claude Code Version

2.1.90 (Claude Code)

Platform

Google Vertex AI

Operating System

Ubuntu/Debian Linux

Terminal/Shell

iTerm2

Additional Information

I am using my macos iterm2 to connect to remote server where claude code runs.