…#53101)

Follow up to #53017

This PR does some significant refactoring of the `follow_tail` feature
in the GPUI list. That's only used by the agent panel's thread view and
given to the height-changing nature of streaming agent responses, we
were seeing some scroll snapping bugs upon scrolling while the thread is
generating. In the process of fixing it, we introduced a
`remeasure_items` method as an alternative to `splice` so that we could
get the remeasurement fix without scroll position changes. We already
had a `remeasure` method that did that for all of the indexes, but we
needed something more scoped out for the agent panel case, so as to not
remeasure the entire list's content on every new streamed token.

Effectively, this ends up reverting what the PR linked above introduced,
but it improved the API in the process.
 
Release Notes:

- N/A

Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>

Refactoring in #53101 broke manual scrolling in many ways. This change
restores it, accounting for the new FollowMode enum.

Follow-up to #53101

In the process of fixing the thread view's scroll experience, we for got
to turn off the follow state tail for functions that power the scroll
buttons in the agent panel.

Release Notes:

- N/A

Follow-up to zed-industries#53101

In the process of fixing the thread view's scroll experience, we for got
to turn off the follow state tail for functions that power the scroll
buttons in the agent panel.

Release Notes:

- N/A

…tion

Port upstream follow_tail refactor (Zed PRs zed-industries#53101, zed-industries#53232) as fresh
feature addition:

- Add FollowMode (pub) + FollowState (internal) enums
- Migrate ListItem::Unmeasured estimated_height → size_hint: Option<Size<Pixels>>
- Add remeasure_items(range) — scoped remeasure preserving size hints
- Add set_follow_mode(), is_following_tail(), scroll_to_end()
- Add follow_state logic to scroll/scroll_by/scroll_to/set_offset_from_scrollbar
- Add auto-scroll + re-engagement in layout_items()
- Add is_following_tail field to ListScrollEvent

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>