fix(channels): fix pin to bottom

[synoet]

Note

Medium Risk
Changes core virtualized scroll pinning and measurement timing for all channels; behavior shifts for users slightly above the bottom (1px vs 50px), though covered by new e2e tests.

Overview
Fixes channel message list bouncing away from the bottom during sends, acks, and layout changes by tightening when the list is considered “pinned” and when row geometry is measured.

NEAR_BOTTOM_THRESHOLD drops from 50px to 1px so append-follow, composer/viewport resize, and saved snapshots share one strict end tolerance. A looser threshold could treat optimistic height estimates (e.g. after ID swap on ack) as still pinned and apply a backward scroll correction.

In ThreadList, useAnimationFrameWithResizeObserver is disabled so ResizeObserver-driven measurements apply in the current frame instead of the next, avoiding stale sizer/composer geometry that could drop the end anchor on send. Row measureElement runs in a queueMicrotask after child effects render Markdown, so the virtualizer does not measure an empty row shell and temporarily shrink the scroll extent.

Docs (sticky-scrolling.md, channels.md) describe the 1px boundary and measurement timing. New Playwright e2e (local-channel-send-scroll.spec.ts) asserts zero bottom presentation violations across consecutive sends, wrapped/tall messages, mobile touch viewports, sends at various gaps from bottom, composer growth, and simulated keyboard viewport shrink.

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📥 Commits

Reviewing files that changed from the base of the PR and between 17a1dc1 and d237f62.

📒 Files selected for processing (5)

• apps/web/src/features/channel/Channel/ThreadList.tsx
• apps/web/src/features/channel/Channel/constants.ts
• apps/web/src/features/channel/docs/sticky-scrolling.md
• apps/web/tests/e2e/local-channel-send-scroll.spec.ts
• docs/AGENT_GUIDE/channels.md

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📝 Summary

Summary by CodeRabbit

• Bug Fixes

  • Improved channel message scrolling to remain pinned to the bottom during consecutive sends, server acknowledgements, composer resizing, and viewport changes.
  • Preserved the reader’s position when scrolling through message history or loading older messages.
  • Improved behavior when returning to the bottom of a channel, ensuring new messages are followed reliably.

• Documentation

  • Updated channel scrolling guidance to reflect the refined bottom-pinning and history-navigation behavior.

Walkthrough

The channel virtualizer now measures resize changes synchronously and remeasures rows in a microtask after child rendering. The near-bottom threshold is 1px. Documentation defines the updated scrolling and measurement behavior. New local Playwright tests cover consecutive sends, server acknowledgements, composer resizing, viewport contraction, and non-pinned scroll stability across desktop and mobile viewports.

Merge Risk: ⚪ Minimal · up to d237f

Channel scrolling now uses a precise bottom threshold and synchronous/deferred measurements to keep pinned views stable while preserving positions when reading history. The covered send and resize scenarios indicate no remaining merge-blocking risk.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title uses the conventional commits format with the fix(channels): prefix, describes the channel pinning fix, and is 32 characters long.
Description check	✅ Passed	The description directly explains the channel scroll-pinning fix, the threshold and measurement changes, documentation updates, and the added end-to-end tests.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Preview: https://synoet-channel-pin-bottom-jank-ce76lm.preview.macro.com/app (fc655ba)