fix(chat): defer scroller resize correction off the ResizeObserver callback

[posthog]

Summary

A single chat session logged the browser warning "ResizeObserver loop completed with undelivered notifications" hundreds of times, and it was also being forwarded into error tracking as a real exception.

Two changes:

• Root cause — the message scroller's ResizeObserver callback (handleResize) wrote viewport.scrollTop synchronously (via scrollToEnd / reanchorToAnchoredMessage), mutating layout inside the same observer delivery — the classic trigger for that warning. The resize-driven scroll correction is now coalesced onto a requestAnimationFrame, so the write lands outside the observer callback. The frame is cancelled on unmount for StrictMode safety, matching the existing state-commit frame handling.
• Noise — the PostHog global error handler now drops the known-benign ResizeObserver loop* warnings, so a stray occurrence can't bury genuine errors in error tracking.

An earlier scroll-stabilization pass didn't stop the loop; deferring the mutation off the observer callback is the direct fix.

Why

One user hit this loop repeatedly in a single chat session while using web access; session replay flagged it a blocking_exception, and the unfiltered global handler was logging every occurrence as an error-tracking issue. It's both a real UX degradation for affected users and clutter that can hide genuine errors.

Testing notes

• pnpm check (format, lint, types) — passes
• pnpm test — 14 passed

The symptom is a browser layout-timing warning surfaced through error tracking rather than a rendered visual glitch; reproducing it live needs a streaming chat session against the backend, which wasn't available in this environment. The requestAnimationFrame deferral is the established mitigation for this specific warning and preserves the existing follow-bottom / anchor behavior (the append/anchor path via the content mutation observer is unchanged).

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Created with PostHog Code from an inbox report.

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[github-actions]

React Doctor found no new issues. 🎉

_{Reviewed by React Doctor for commit 8ad9b4f.}