## Summary

Expose whether the scrollbar is currently being dragged via a new
`is_scrollbar_dragging()` method on `ListState`.

This returns `true` between `scrollbar_drag_started()` and
`scrollbar_drag_ended()` calls. It lets consumers distinguish scrollbar
drags from wheel/trackpad scrolls, which is useful for suppressing
auto-scroll or follow-tail behavior during manual scrollbar positioning.

## Test plan

- [x] Code review — one-liner accessor, delegates to existing
`scrollbar_drag_start_height` field
- [ ] Verify compilation with `cargo check -p gpui`

Release Notes:

- N/A

🤖 Generated with [Claude Code](https://claude.com/claude-code)