This PR fixes some cases where scrolling is choppy due to a user
inputting a scroll event while a list state is going through a
remeasure. Currently, the list state would ignore the user's scroll and
fall back to the pending scroll position, this PR fixes this by rebasing
the pending scroll onto the user's new scroll position.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- agent_panel: Improve scroll smoothness while a response is streaming