Fix idle momentum end on iOS (#58658)

[Abbondanzo]

Summary:

Unmounting an idle ScrollView could incorrectly dispatch onMomentumScrollEnd because a non-tracking scroll view was treated as moving. Check UIScrollView.isDecelerating before the view leaves its window so the event is emitted only when momentum is active.

Add native regression coverage plus a shared RNTester Maestro flow. The flow runs on Android and iOS and verifies an idle unmount emits no momentum event, one fling emits exactly one begin/end pair, and a later unmount does not increment the count.

Changelog:
[iOS][Fixed] - Prevent idle ScrollViews from firing onMomentumScrollEnd during unmount

Differential Revision: D121477503

[meta-codesync]

@Abbondanzo has exported this pull request. If you are a Meta employee, you can view the originating Diff in D121477503.

[Abbondanzo]

/review

[github-actions]

🤖 AI code review

Decision: Ready for human review (with comments)

Overall PR risk: Medium. The change modifies existing behavior in iOS ScrollView momentum-end emission on unmount and VirtualizedList onEndReached and onStartReached gating. The affected surface covers ScrollView and FlatList users, with narrow guards and direct rollback by revert. The kept finding shows the tester Scroll to start path never enables recording, so first use leaves output at running, while the broader plausible break is missed or extra momentum-end and edge callbacks.

🟡 Warning (1)

• scrollToStart never enables event recording — packages/rn-tester/js/examples/FlatList/FlatList-onEndReached.js:99 (correctness) · id:a2b141266792
  Confidence: High — direct trace through the changed handlers.
  Impact if shipped: Medium — the tester button hangs on first use.
  Suggested remediation: Add recordEventsRef.current = true; at the start of scrollToStart, matching scrollToEnd.

  Evidence and reasoning

  scrollToEnd sets recordEventsRef to true. scrollToStart does not set it. All handlers return early when recordEventsRef is false. A user taps Scroll to start first. The list scrolls. The handlers ignore the events. The output stays at running. No later event reports it.

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This review is advisory — it never blocks a merge and never auto-approves.