fix(web): correct thread scroll-to-bottom pill behavior

[UtkarshUsername]

Fixes #2119

What Changed

Fixes the pill showing when opening an existing thread that is already at the bottom

Why

When a thread is opened, the "Scroll to bottom" button was always shown even when the chat is at the bottom. Clicking it did nothing.

UI Changes

Before:

scroll-before.mp4

After:

scroll-after.mp4

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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Note

Low Risk
Low risk UI-behavior change limited to client-side scroll state tracking and a new initial snap-to-bottom path, with added tests to guard regressions.

Overview
Fixes the scroll-to-bottom pill incorrectly appearing when opening an already-bottomed thread by tracking programmatic scrolls and suppressing transient isAtEnd updates while those scrolls are in flight.

MessagesTimeline now performs a RAF-scheduled initial scrollToEnd when rows first appear (and reschedules if rows change before the frame runs), and derives isAtEnd directly from scroll event metrics when available. ChatView centralizes scroll-to-end behavior via beginProgrammaticScroll/scrollToEnd, clears pending state on thread switch/unmount, and updates optimistic-send flows to use the new helper.

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Note

Fix scroll-to-bottom pill visibility during programmatic scrolls in thread view

• Introduces programmatic scroll tracking in ChatView so that onIsAtEnd changes are suppressed while a programmatic scroll is in flight, preventing the scroll-to-bottom pill from flashing during initial thread load or explicit scroll-to-end actions.
• Replaces the previous initial scroll logic in MessagesTimeline with a RAF-based snap-to-bottom on mount, rescheduled if rows change before the frame runs.
• Adds a deriveIsAtEndFromScrollEvent helper to compute at-end status from native scroll event metrics (within 2px threshold), reducing reliance on LegendList state queries.
• Clears pending programmatic scroll state on thread switch and component unmount.
• Behavioral Change: optimistic message insertion now calls scrollToEnd(false) before updating, replacing direct isAtEndRef/UI resets.

^{Macroscope summarized be1a2de.}

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[macroscopeapp]

Approvability

Verdict: Approved

Self-contained UI bug fix for scroll-to-bottom pill behavior in chat threads. Changes add tracking for programmatic scrolls to prevent the pill from incorrectly appearing during automated scroll operations. Includes comprehensive tests and touches no sensitive areas.

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Dismissing prior approval to re-evaluate 6732004

Dismissing prior approval to re-evaluate 245349f

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