Fix scrollView state contentOffset not being in sync with scrollView contentOffset

Summary:
Changelog: [Internal]

# Problem
`scrollView.state.contentOffset` was not in sync with actual `contentOffset` in case `contentOffset` is changed programatically.

# Solution
Add a flag `_isUserTriggeredScrolling` that indicates whether the current scroll is triggered by user or not. In case it isn't, update state.

Reviewed By: shergin

Differential Revision: D20098161

fbshipit-source-id: 021d916e7a45a24095a47bb8f84d1102226b672a

React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm

@@ -48,6 +48,11 @@ static void RCTSendPaperScrollEvent_DEPRECATED(UIScrollView *scrollView, NSInteg
   CGSize _contentSize;
   NSTimeInterval _lastScrollEventDispatchTime;
   NSTimeInterval _scrollEventThrottle;
+  // Flag indicating whether the scrolling that is currently happening
+  // is triggered by user or not.
+  // This helps to only update state from `scrollViewDidScroll` in case
+  // some other part of the system scrolls scroll view.
+  BOOL _isUserTriggeredScrolling;
 }
 
 + (RCTScrollViewComponentView *_Nullable)findScrollViewComponentViewForView:(UIView *)view
@@ -67,6 +72,7 @@ static void RCTSendPaperScrollEvent_DEPRECATED(UIScrollView *scrollView, NSInteg
     _scrollView = [[RCTEnhancedScrollView alloc] initWithFrame:self.bounds];
     _scrollView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
     _scrollView.delaysContentTouches = NO;
+    _isUserTriggeredScrolling = NO;
     [self addSubview:_scrollView];
 
     _containerView = [[UIView alloc] initWithFrame:CGRectZero];
@@ -204,8 +210,10 @@ static void RCTSendPaperScrollEvent_DEPRECATED(UIScrollView *scrollView, NSInteg
 
 - (void)_updateStateWithContentOffset
 {
+  if (!_state) {
+    return;
+  }
   auto contentOffset = RCTPointFromCGPoint(_scrollView.contentOffset);
-
   _state->updateState([contentOffset](ScrollViewShadowNode::ConcreteState::Data const &data) {
     auto newData = data;
     newData.contentOffset = contentOffset;
@@ -217,6 +225,7 @@ static void RCTSendPaperScrollEvent_DEPRECATED(UIScrollView *scrollView, NSInteg
 {
   _scrollView.contentOffset = CGPointZero;
   _state.reset();
+  _isUserTriggeredScrolling = NO;
   [super prepareForRecycle];
 }
 
@@ -224,6 +233,9 @@ static void RCTSendPaperScrollEvent_DEPRECATED(UIScrollView *scrollView, NSInteg
 
 - (void)scrollViewDidScroll:(UIScrollView *)scrollView
 {
+  if (!_isUserTriggeredScrolling) {
+    [self _updateStateWithContentOffset];
+  }
   if (!_eventEmitter) {
     return;
   }
@@ -243,6 +255,18 @@ static void RCTSendPaperScrollEvent_DEPRECATED(UIScrollView *scrollView, NSInteg
   [self scrollViewDidScroll:scrollView];
 }
 
+- (BOOL)scrollViewShouldScrollToTop:(UIScrollView *)scrollView
+{
+  _isUserTriggeredScrolling = YES;
+  return YES;
+}
+
+- (void)scrollViewDidScrollToTop:(UIScrollView *)scrollView
+{
+  _isUserTriggeredScrolling = NO;
+  [self _updateStateWithContentOffset];
+}
+
 - (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
 {
   [self _forceDispatchNextScrollEvent];
@@ -252,6 +276,7 @@ static void RCTSendPaperScrollEvent_DEPRECATED(UIScrollView *scrollView, NSInteg
   }
 
   std::static_pointer_cast<ScrollViewEventEmitter const>(_eventEmitter)->onScrollBeginDrag([self _scrollViewMetrics]);
+  _isUserTriggeredScrolling = YES;
 }
 
 - (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
@@ -288,6 +313,7 @@ static void RCTSendPaperScrollEvent_DEPRECATED(UIScrollView *scrollView, NSInteg
 
   std::static_pointer_cast<ScrollViewEventEmitter const>(_eventEmitter)->onMomentumScrollEnd([self _scrollViewMetrics]);
   [self _updateStateWithContentOffset];
+  _isUserTriggeredScrolling = NO;
 }
 
 - (void)scrollViewDidEndScrollingAnimation:(UIScrollView *)scrollView