Restore scroll position when scroll view is hidden and shown

Summary:
ScrollViews don't properly maintain position where they are hidden and shown. On iOS, when a UIScrollView (or its ancestor) is hidden, its scroll position is set to 0 (its window also becomes nil). When it is shown again, its scroll position is not restored.

When a scroll is attempted when the scroll view is hidden, we keep track of the last known offset before it was hidden. Then, in updateLayoutMetrics (which is triggered when the view is shown), we apply the pending offset if there is one. This is [consistent with Android's behavior in ReactScrollView.java](https://www.internalfb.com/code/fbsource/[2930f8c146af62ad63673c8d34e9876b77634c05]/xplat/js/react-native-github/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java?lines=289).

Changelog:
[Internal][Fixed] - In onLayoutChange, only scroll if the view is shown and the content view is ready

Reviewed By: cipolleschi

Differential Revision: D42815359

fbshipit-source-id: 4b209c1e54edf3f5c0bea902b48450a1a2e9661a

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

@@ -25,6 +25,12 @@
 
 using namespace facebook::react;
 
+struct PendingOffset {
+  bool isPending;
+  CGPoint offset;
+  CGPoint lastOffset;
+};
+
 static CGFloat const kClippingLeeway = 44.0;
 
 static UIScrollViewKeyboardDismissMode RCTUIKeyboardDismissModeFromProps(ScrollViewProps const &props)
@@ -99,6 +105,8 @@ @implementation RCTScrollViewComponentView {
   BOOL _shouldUpdateContentInsetAdjustmentBehavior;
 
   CGPoint _contentOffsetWhenClipped;
+
+  PendingOffset _pendingOffset;
 }
 
 + (RCTScrollViewComponentView *_Nullable)findScrollViewComponentViewForView:(UIView *)view
@@ -173,6 +181,12 @@ - (void)updateLayoutMetrics:(const LayoutMetrics &)layoutMetrics
     _containerView.transform = transform;
     _scrollView.transform = transform;
   }
+
+  // If there is a pending offset, apply it
+  if (_pendingOffset.isPending) {
+    [self scrollTo:_pendingOffset.offset.x y:_pendingOffset.offset.y animated:false];
+    _pendingOffset.isPending = false;
+  }
 }
 
 - (void)updateProps:(Props::Shared const &)props oldProps:(Props::Shared const &)oldProps
@@ -421,6 +435,13 @@ - (void)scrollViewDidScroll:(UIScrollView *)scrollView
     [self _updateStateWithContentOffset];
   }
 
+  // If the view is hidden, then set as pending offset. Apply it later on
+  // updateLayoutMetrics.
+  if (_scrollView.window == nil && !_pendingOffset.isPending) {
+    _pendingOffset.offset = _pendingOffset.lastOffset;
+    _pendingOffset.isPending = true;
+  }
+
   NSTimeInterval now = CACurrentMediaTime();
   if ((_lastScrollEventDispatchTime == 0) || (now - _lastScrollEventDispatchTime > _scrollEventThrottle)) {
     _lastScrollEventDispatchTime = now;
@@ -431,6 +452,8 @@ - (void)scrollViewDidScroll:(UIScrollView *)scrollView
     RCTSendScrollEventForNativeAnimations_DEPRECATED(scrollView, self.tag);
   }
 
+  _pendingOffset.lastOffset = _scrollView.contentOffset;
+
   [self _remountChildrenIfNeeded];
 }