fix: reset IntersectionObserver on 0→non-zero root width transition

[Copilot]

The recovery path for Safari's zero-width root race condition was a no-op. observeSubscribedElements() called observer.observe(el) on elements already being observed — which the IntersectionObserver spec defines as a no-op — so no fresh initial batch was ever delivered. Items that had collapsed during the bad zero-width batch stayed stuck as "not overflowing."

Changes

OverflowObserverProvider.tsx

• On the 0 → non-zero root width transition, fully reset the shared IntersectionObserver before re-observing:

  observerRef.current?.disconnect()
  observerRef.current = null
  observerRootRef.current = null
  observedElementsRef.current.clear()
  observeSubscribedElements() // getObserver() now builds a fresh IO instance

  Nulling both refs forces getObserver() to construct a new IntersectionObserver, which delivers a correct initial batch for every subscribed element. No new persistent observers are introduced.

useOverflowObserver.test.tsx

• Added disconnected: boolean tracking to MockIntersectionObserver.disconnect().
• Strengthened the transition test: vi.spyOn(HTMLElement.prototype, 'clientWidth', 'get') is applied before render() so the ResizeObserver effect captures lastWidth = 0 (in a real Chromium environment the <ul> has clientWidth > 0 without this). The test now explicitly asserts that the old observer was disconnected and a new instance was created — it fails against the prior no-op implementation.

Changelog

New

Changed

• OverflowObserverProvider: on the 0→non-zero root width transition, the shared IntersectionObserver is now fully disconnected and rebuilt to guarantee a fresh observation batch.

Removed

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

The updated test verifies the contract directly: it fails if disconnect() is not called on the prior observer instance and fails if no new IntersectionObserver instance is created on the transition. The zero-width guard test and sub-pixel threshold test are unchanged and still pass.

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