Add MC to disable onScroll requiring scrollEventThrottle on iOS. (#38742)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/38742

If you try to add an `onScroll` event listener on iOS, you will get a warning that you must additionally add a `scrollEventThrottle`, otherwise you will only receive a single event throughout the duration of the gesturing.

Values under 17ms are unthrottled, so it is common to see components passing magic values `16` and `0.0001` to ask for full events.

This behavior is inconsistent with Android, Web, and Windows.

This change runs an experiment on iOS to continuously fire scroll events to users of `onScroll` without `scrollEventThrottle`. Because there are warnings already, I think this case will be rare, and any code written for Android will already have to be written to be compatible with it.

I intentionally left the warning in for now, so that folks don't add new code relying on this QE while it is being tested.

Changelog: [Internal]

Reviewed By: yungsters

Differential Revision: D47975260

fbshipit-source-id: 8f182c5514cc557e1804586e8cbc68fc54502d83

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

@@ -442,7 +442,8 @@ static void RCTSendScrollEventForNativeAnimations_DEPRECATED(UIScrollView *scrol
   }
 
   NSTimeInterval now = CACurrentMediaTime();
-  if ((_lastScrollEventDispatchTime == 0) || (now - _lastScrollEventDispatchTime > _scrollEventThrottle)) {
+  if (CoreFeatures::disableScrollEventThrottleRequirement || (_lastScrollEventDispatchTime == 0) ||
+      (now - _lastScrollEventDispatchTime > _scrollEventThrottle)) {
     _lastScrollEventDispatchTime = now;
     if (_eventEmitter) {
       static_cast<ScrollViewEventEmitter const &>(*_eventEmitter).onScroll([self _scrollViewMetrics]);

packages/react-native/React/Fabric/RCTSurfacePresenter.mm

@@ -280,6 +280,10 @@ static BackgroundExecutor RCTGetBackgroundExecutor()
     CoreFeatures::enableMountHooks = true;
   }
 
+  if (reactNativeConfig && reactNativeConfig->getBool("react_fabric:disable_scroll_event_throttle_requirement")) {
+    CoreFeatures::disableScrollEventThrottleRequirement = true;
+  }
+
   auto componentRegistryFactory =
       [factory = wrapManagedObject(_mountingManager.componentViewRegistry.componentViewFactory)](
           EventDispatcher::Weak const &eventDispatcher, ContextContainer::Shared const &contextContainer) {

packages/react-native/ReactCommon/react/utils/CoreFeatures.cpp

@@ -19,5 +19,6 @@ bool CoreFeatures::enableGranularScrollViewStateUpdatesIOS = false;
 bool CoreFeatures::enableMountHooks = false;
 bool CoreFeatures::doNotSwapLeftAndRightOnAndroidInLTR = false;
 bool CoreFeatures::enableCleanParagraphYogaNode = false;
+bool CoreFeatures::disableScrollEventThrottleRequirement = false;
 
 } // namespace facebook::react

packages/react-native/ReactCommon/react/utils/CoreFeatures.h

@@ -56,6 +56,10 @@ class CoreFeatures {
 
   // Clean yoga node when <Text /> does not change.
   static bool enableCleanParagraphYogaNode;
+
+  // Fire `onScroll` events continuously on iOS without a `scrollEventThrottle`
+  // props, and provide continuous `onScroll` upates like other platforms.
+  static bool disableScrollEventThrottleRequirement;
 };
 
 } // namespace facebook::react