Fix idle momentum end on iOS

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 `ScrollView`s from firing `onMomentumScrollEnd` during unmount

Differential Revision: D121477503

Author: Abbondanzo

packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm

@@ -844,19 +844,19 @@ - (void)scrollViewDidEndScrollingAnimation:(UIScrollView *)scrollView
   [self _handleFinishedScrolling:scrollView];
 }
 
-- (void)didMoveToWindow
+- (void)willMoveToWindow:(UIWindow *)newWindow
 {
-  [super didMoveToWindow];
+  [super willMoveToWindow:newWindow];
 
-  if (!self.window) {
+  if (!newWindow) {
     // The view is being removed, ensure that the scroll end event is dispatched
     [self _handleScrollEndIfNeeded];
   }
 }
 
 - (void)_handleScrollEndIfNeeded
 {
-  if (_scrollView.isDecelerating || !_scrollView.isTracking) {
+  if (_scrollView.isDecelerating) {
     if (!_eventEmitter) {
       return;
     }

packages/react-native/React/Tests/Mounting/RCTScrollViewComponentViewTests.mm

@@ -7,10 +7,13 @@
 
 #import <React/RCTScrollViewComponentView.h>
 #import <XCTest/XCTest.h>
+#import <react/renderer/components/scrollview/ScrollViewEventEmitter.h>
 #import <react/renderer/components/scrollview/ScrollViewProps.h>
 #import <react/renderer/components/scrollview/ScrollViewShadowNode.h>
 
+using facebook::react::EventDispatcher;
 using facebook::react::Props;
+using facebook::react::ScrollViewEventEmitter;
 using facebook::react::ScrollViewProps;
 using facebook::react::ScrollViewShadowNode;
 
@@ -61,6 +64,21 @@ - (void)testAutomaticallyAdjustKeyboardInsetsAcrossRecycling
   XCTAssertEqual(view.scrollView.contentInset.bottom, 50);
 }
 
+- (void)testUnmountingIdleScrollViewDoesNotEndMomentum
+{
+  UIWindow *window = [[UIWindow alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];
+  RCTScrollViewComponentView *view = [[RCTScrollViewComponentView alloc] initWithFrame:window.bounds];
+  [window addSubview:view];
+
+  auto eventEmitter = std::make_shared<ScrollViewEventEmitter>(nullptr, EventDispatcher::Weak{});
+  [view updateEventEmitter:eventEmitter];
+  [view setValue:@YES forKey:@"isUserTriggeredScrolling"];
+
+  [view removeFromSuperview];
+
+  XCTAssertTrue([[view valueForKey:@"isUserTriggeredScrolling"] boolValue]);
+}
+
 @end
 
 #endif

packages/rn-tester/.maestro/scrollview-momentum-events.yml

@@ -0,0 +1,54 @@
+appId: ${APP_ID} # iOS: com.meta.RNTester.localDevelopment | Android: com.facebook.react.uiapp
+---
+- launchApp
+- stopApp
+- openLink: rntester://example/ScrollViewExample/onMomentumScroll
+- runFlow: ./helpers/confirm-open-link.yml
+- extendedWaitUntil:
+    visible:
+      id: 'momentum-scroll-view'
+    timeout: 120000
+- assertVisible:
+    id: 'momentum-scroll-begin-count'
+    text: 'onMomentumScrollBegin called 0 times'
+- assertVisible:
+    id: 'momentum-scroll-end-count'
+    text: 'onMomentumScrollEnd called 0 times'
+# Removing an idle ScrollView must not synthesize a momentum-end event.
+- tapOn:
+    id: 'toggle-momentum-scroll-view'
+- assertNotVisible:
+    id: 'momentum-scroll-view'
+- waitForAnimationToEnd:
+    timeout: 1000
+- assertVisible:
+    id: 'momentum-scroll-end-count'
+    text: 'onMomentumScrollEnd called 0 times'
+- tapOn:
+    id: 'toggle-momentum-scroll-view'
+- assertVisible:
+    id: 'momentum-scroll-view'
+# One momentum scroll must produce exactly one begin and one end event.
+- swipe:
+    start: 50%, 55%
+    end: 50%, 25%
+    speed: fast
+- waitForAnimationToEnd:
+    timeout: 5000
+- assertVisible:
+    id: 'momentum-scroll-begin-count'
+    text: 'onMomentumScrollBegin called 1 times'
+- assertVisible:
+    id: 'momentum-scroll-end-count'
+    text: 'onMomentumScrollEnd called 1 times'
+# Unmounting after momentum has ended must not increment the count again.
+- tapOn:
+    id: 'toggle-momentum-scroll-view'
+- waitForAnimationToEnd:
+    timeout: 1000
+- assertVisible:
+    id: 'momentum-scroll-begin-count'
+    text: 'onMomentumScrollBegin called 1 times'
+- assertVisible:
+    id: 'momentum-scroll-end-count'
+    text: 'onMomentumScrollEnd called 1 times'

packages/rn-tester/js/examples/ScrollView/ScrollViewExample.js

@@ -382,9 +382,10 @@ const examples: Array<RNTesterModuleExample> = [
     },
   },
   {
+    name: 'onMomentumScroll',
     title: '<ScrollView> OnMomentumScroll\n',
     description:
-      'An alert will be called when the momentum scroll starts or ends.',
+      'Counts momentum scroll events and supports unmounting the ScrollView.',
     render(): React.Node {
       return <OnMomentumScroll />;
     },
@@ -920,17 +921,37 @@ const OnScrollOptions = () => {
 };
 
 const OnMomentumScroll = () => {
-  const [scroll, setScroll] = useState('none');
+  const [scrollViewMounted, setScrollViewMounted] = useState(true);
+  const [momentumScrollBeginCount, setMomentumScrollBeginCount] = useState(0);
+  const [momentumScrollEndCount, setMomentumScrollEndCount] = useState(0);
+
   return (
     <View>
-      <RNTesterText>Scroll State: {scroll}</RNTesterText>
-      <ScrollView
-        style={[styles.scrollView, {height: 200}]}
-        onMomentumScrollBegin={() => setScroll('onMomentumScrollBegin')}
-        onMomentumScrollEnd={() => setScroll('onMomentumScrollEnd')}
-        nestedScrollEnabled>
-        {ITEMS.map(createItemRow)}
-      </ScrollView>
+      <RNTesterText testID="momentum-scroll-begin-count">
+        onMomentumScrollBegin called {momentumScrollBeginCount} times
+      </RNTesterText>
+      <RNTesterText testID="momentum-scroll-end-count">
+        onMomentumScrollEnd called {momentumScrollEndCount} times
+      </RNTesterText>
+      <Button
+        label={scrollViewMounted ? 'Unmount ScrollView' : 'Mount ScrollView'}
+        onPress={() => setScrollViewMounted(mounted => !mounted)}
+        testID="toggle-momentum-scroll-view"
+      />
+      {scrollViewMounted ? (
+        <ScrollView
+          style={styles.scrollView}
+          onMomentumScrollBegin={() =>
+            setMomentumScrollBeginCount(count => count + 1)
+          }
+          onMomentumScrollEnd={() =>
+            setMomentumScrollEndCount(count => count + 1)
+          }
+          testID="momentum-scroll-view"
+          nestedScrollEnabled>
+          {ITEMS.map(createItemRow)}
+        </ScrollView>
+      ) : null}
     </View>
   );
 };