[BUG] Mouse selection causes input box border lines to wrap and display jumps

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 using mouse selection inside Claude Code running in tmux (with set -g mouse on),
the border lines of the input area extend beyond the terminal width, causing them to
wrap to the next line. This creates a visible "jump" of ~2 rows in the display.

What Should Happen?

It should not jump :)

<img width="1152" height="135" alt="Image" src="https://github.com/user-attachments/assets/a7edd8db-5bcc-4aee-84f7-c26b2050358e" />

Environment:

• Claude Code (CLI)
• Terminal: Ghostty
• tmux with mouse mode enabled
• macOS, Apple Magic Trackpad

Error Messages/Logs

Steps to Reproduce

Steps to reproduce:

1. Run Claude Code inside a tmux session with set -g mouse on
2. Try to select text with the mouse in or near the input area
3. The border lines of the input box wrap past the terminal edge, causing the display

to shift/jump by 2 rows

Workaround: Holding Shift while selecting bypasses tmux mouse handling and avoids the
issue.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.100

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

_No response_