Derive overflow from root-scoped IntersectionObserver entries instead of offsetTop reads

[Copilot]

Follow-up to the shared-overflow-observer work. The shared IntersectionObserver in the descendant registry now derives overflow directly from observer entries (isIntersecting / intersectionRatio) and is scoped to the clipping container via a new root, instead of being used only as a trigger to re-read offsetTop > 0. This closes the layout-thrash path (no synchronous offsetTop reads) and removes the per-item IntersectionObserver fallback.

Base branch is underline-nav-shared-observer-coalesced-rebuild, so this diff is scoped to the changes on top of that work — not main.

Changelog

New

• ProviderProps accepts a rootRef that is passed to the shared IntersectionObserver as its root, scoping intersection to the actual clipping container (the ActionBar overflow container / UnderlineNav item list).
• getIsOverflowing(entry) derives overflow state from the observed entry: any target that isn't fully visible (!isIntersecting || intersectionRatio < 1) counts as overflowing, guarding sub-pixel boundary cases.

Changed

• The shared observer callback now computes isOverflowing per entry and fans that boolean out to subscribers, rather than firing a bare onChange() that forced an offsetTop re-read.
• ObserveFn / useRegisterOverflowObserver now propagate the overflow boolean (onOverflowChange(isOverflowing)); useSyncExternalStore reads a tracked isOverflowingRef instead of ref.current.offsetTop > 0.
• Observer is created with {root, threshold: [0, 1]}; when the root element changes, the provider re-observes all currently-subscribed elements against the new observer instance (observeSubscribedElements + an isomorphic layout effect).
• ActionBar and UnderlineNav pass rootRef to the registry Provider (overflowContainerRef / listRef).
• The registry overflow option is simplified from {threshold?: number} to an object flag — the explicit threshold: 1 / threshold: 0.75 configs on ActionBar and UnderlineNav are dropped, since overflow is now read from the entry rather than tuned via threshold.
• SSR/availability guard extracted to supportsIntersectionObserver().

Removed

• Per-item local IntersectionObserver fallback: the hook now tracks overflow only through the provider's shared observer and is inert (false) when no shared observer is configured.
• All synchronous offsetTop layout reads from the overflow path (and the related code comments describing the offsetTop-trigger model).
• The 0.75 threshold workaround comment in ActionBar (no longer needed without threshold tuning).

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Internal-only refactor; no public API or behavior change. Overflow detection moves from an offsetTop-trigger model to reading the observed IntersectionObserverEntry directly.

packages/react/src/utils/__tests__/descendant-registry.test.tsx is updated to match:

• The mock observer now captures its options and exposes emit(entries) so tests can drive real entry shapes (target / isIntersecting / intersectionRatio) instead of a bare trigger().
• New assertion that the single shared observer is created with the provided root element.
• New test that an item updates from the observed entry without reading layout — offsetTop is stubbed to throw to prove it is never read.
• Existing fan-out, unobserve-on-unmount, and disconnect-on-provider-unmount coverage is retained (plus an explicit unobserve assertion).

Perf characteristics worth noting during review:

• Shared observer, coalesced microtask rebuilds, and per-item useSyncExternalStore are all preserved — siblings don't re-render when one item's overflow flips.
• Overflow is now derived from observer entries with no synchronous offsetTop/layout reads, so the layout-thrash path stays closed; the fallback removal prevents reintroducing an offsetTop-style local observer.
• The registry-rebuild cascade (key bump forcing re-registration on membership/order change) is the main remaining cost and is unchanged here.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

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