fix(thread): stop ChatThread snapping to top on prompt submit

[frankh]

Problem

In the new ChatThread view (behind useNewChatThread):

1. Submitting a prompt in a conversation with history snapped the scroll position back to the top of the conversation on every prompt.
2. Stick-to-bottom was unreliable: submits and streaming replies often didn't scroll the view, and fast streams (large blocks) dropped out of follow mid-reply.

Root causes

The snap. quill's message scroller reacts to childList mutations on the scroller content by comparing direct-child counts. When a prompt is sent, the optimistic user bubble (optimistic-… id) is swapped for the echoed real prompt (turn-…-user id) in a single React commit — a mutation where the child count doesn't change. In that case the engine falls into a re-anchor branch that scans all rows from the top for the first data-scroll-anchor element it hasn't scrolled to before. Historical user messages never qualify as handled, so it scrolls to the first user message in the conversation. A second trigger of the same branch: a lone tool marker collapsing into a tool_group row (1 row → 1 row, key change) mid-response.

Unreliable following. The engine only auto-follows in following-bottom mode, which it re-enters solely within scrollEdgeThreshold (default 8px) of the exact bottom — with the thread's bottom padding the view is rarely that close, and any stray trackpad wheel exits follow mode. Additionally, each engine autoscroll is guarded by a 180ms grace window; a large streamed block (heavy markdown render) can jank past it, making the engine observe "content below the fold while not autoscrolling" and demote itself to free-scrolling mid-reply.

Fix

All in ChatThread.tsx:

• Key user-message rows by occurrence index (user-turn-N) instead of item.id, so the optimistic→real swap reuses the same DOM element and the buggy re-anchor branch never fires.
• Reuse the first tool's item id as the tool_group id, so a lone marker growing into a group keeps its key/element.
• ThreadAutoFollow: pin the view to the bottom from prompt submit until the user scrolls away — scroll to end on submit (which also re-engages the engine's following-bottom), and while armed re-issue scrollToEnd whenever content slips below the fold, recapturing follow after stream bursts. User scroll intent (wheel/touch/pointer/keys) disarms.
• Widen scrollEdgeThreshold to 100px so near-bottom counts as at-bottom for the engine's own follow-mode recapture.

Follow-up: the underlying count-neutral re-anchor bug should also be fixed upstream in @posthog/quill's message scroller.

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Created with PostHog Code

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 8867aec.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(thread): stop ChatThread snapping to..." | Re-trigger Greptile}

[github-actions]

No showstoppers — this is a focused, self-contained UI scroll-behavior fix with no data model, API contract, or business logic changes. The tool_group id simplification is safe (no key collision), the new ScrollToEndOnSubmit component uses correct React patterns, and the wider scroll threshold is a reasonable UX improvement.

New commits pushed (delta classified non_trivial_delta) — stamphog approval dismissed; re-review running automatically.

[github-actions]

No showstoppers — this is a self-contained UI scroll-behavior fix with no data model, API contract, business logic, or security implications.