[BUG] /mcp list viewport doesn't follow selection when running inside tmux

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 running Claude Code inside tmux (with set -g mouse on), the /mcp interactive
list doesn't scroll the viewport to follow the selected item.

Scrolling with the mouse wheel (or arrow keys) moves the selection highlight through
the list items, but the visible area stays fixed — so the selected item quickly moves
off-screen. There is also no scrollbar visible on the right side of the list.
Same issue with keyboard scrolling.

Outside tmux, the same list works as expected.

Environment:

• Linux (headnode, not desktop)
• tmux with set -g mouse on
• TERM=tmux-256color
• default-terminal set to tmux-256color

What Should Happen?

The view should move with the selection and there should be a scrollbar on the right for manual scrolling through the long list of mcps

Error Messages/Logs

Steps to Reproduce

1. Start a tmux session (tmux new)
2. Launch Claude Code (claude)
3. Type /mcp
4. The MCP server list appears — try scrolling down with mouse wheel or arrow keys
5. The selection highlight moves to items below the visible area, but the viewport

stays fixed at the top — the selected item is not scrolled into view

1. Exit tmux and repeat steps 2-4 — scrolling works correctly outside tmux

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.50

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

xfce

Additional Information

_No response_