[BUG] Scroll position incorrect on startup, auto-jumps to top when scrolling (Windows PowerShell)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

Claude Code has incorrect scroll position behavior on Windows PowerShell:

1. On startup: Scrollbar is positioned at the top instead of bottom, requiring manual scrolling to see conversation content
2. When scrolling up: After scrolling up 3 times with mouse wheel, the view automatically jumps back to the top
3. When window is resized: Narrowing the window causes blank space to appear, and content is only visible after scrolling up

This makes it very difficult to review conversation history and work with the CLI.

What Should Happen?

1. Scrollbar should be at the bottom on startup (showing latest content)
2. Scrolling up should work smoothly without auto-jumping to top
3. Window resizing should not cause blank space or scroll position issues

Steps to Reproduce

1. Start Claude Code in PowerShell 7.5: claude
2. Observe scrollbar is at the top, not bottom
3. Scroll up with mouse wheel 3 times
4. Result: View automatically jumps to top
5. Resize window to be narrower
6. Result: Blank space appears, need to scroll up to see content

Environment

• Claude Code Version: 2.1.37
• Operating System: Windows (MSYS_NT-10.0-26220)
• Terminal: PowerShell 7.5.4
• Platform: Anthropic API

Additional Information

• Issue persists even with custom statusLine disabled
• Issue occurs regardless of statusLine content length
• Appears to be a TUI rendering/viewport calculation issue
• Using custom statusLine: ccline-enhanced.js (but issue occurs without it too)

Screenshots

The scrollbar is positioned at the top on startup, and window narrowing causes layout issues with blank space.

Is this a regression?

Unknown - may have existed in previous versions

Impact

High - Makes it difficult to:

• Review conversation history
• Work with long responses
• Use Claude Code in narrow terminal windows