[BUG] Vertical scrollbar disappears intermittently when using background agents (reproducible across multiple terminals)

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?

Environment:

• OS: Windows 10 Home (10.0.19045)
• Tested terminals: Git Bash (mintty), Command Prompt, PowerShell...
• Claude Code version: 2.1.161 (Claude Code)

Description:
The vertical scrollbar does not appear during sessions using agents (claude agents). The problem is reproducible in several terminal emulators, suggesting that the root cause lies in the rendering of Claude Code's agent mode, and not in the terminal itself.

Without the scrollbar, it is not possible to scroll up to read previous replies. Shift+PageUp also does not work.

<img width="1920" height="1080" alt="Image" src="https://github.com/user-attachments/assets/36855c4b-6274-4a14-a6c2-d93af216f449" />
<img width="1920" height="1080" alt="Image" src="https://github.com/user-attachments/assets/08ca7c8f-40bf-4247-9e25-9630ba7e4c55" />

What Should Happen?

Expected:
Scrollbar always visible and functional regardless of agent activity

Actual:
Scrollbar disappears after background agent usage, making it impossible to review previous conversation output

Error Messages/Logs

Steps to Reproduce

Steps to reproduce:

1. Start a "claude agents" session on any terminal.
2. Run one or more tasks from the Claude agent.
3. Within an agent window, continue the conversation through several message exchanges until the entire screen is filled with new messages.
4. The scroll bar does not appear, and it becomes impossible to scroll to previous messages.

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.161 (Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

Additional Notes:

• The problem does NOT occur when calling the regular "claude" in the terminal, but only happens when calling the "claude agents" mode in the terminal.
• Increasing the terminal's scroll buffer (tested up to 100,000 lines) does not solve the problem.
• Restarting the terminal does not solve the problem.