From 4f8a8ce316494db99b19f6c8db6b0c1e7b6500d9 Mon Sep 17 00:00:00 2001
From: Nick Gerleman <ngerlem@meta.com>
Date: Mon, 21 Aug 2023 18:03:43 -0700
Subject: [PATCH] Fix inverted `contentOffset` in scroll events in RTL (#39031)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39031

`UIScrollView` `contentOffset` is flow-relative, so `x` is relative to the right edge of the screen. This coordinate space disagrees with layout events, `scrollTo` coordinates, and other platforms.

This applies the same logic we use for inverting `scrollTo` coordinates to invert `contentOffset` in scroll events, in both Paper and Fabric. We then remove the iOS specific workaround we have in VirtualizedList.

I did not test `contentInset` as part of this, whose structure has explicit edges like `left` and `right`.

Changelog:
[iOS][Fixed] - Fix inverted `contentOffset` in scroll events in RTL

Reviewed By: rozele

Differential Revision: D48379915

fbshipit-source-id: 8a9cbb01608e79ef3b179a76fbe3997a0cd23845
---
 .../ScrollView/RCTScrollViewComponentView.mm       |  5 +++++
 .../React/Views/ScrollView/RCTScrollView.m         |  8 +++++++-
 .../virtualized-lists/Lists/VirtualizedList.js     | 14 +++++++-------
 3 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm b/packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm
index b8894132b7..add5f743eb 100644
--- a/packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm
+++ b/packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm
@@ -390,6 +390,11 @@ static void RCTSendScrollEventForNativeAnimations_DEPRECATED(UIScrollView *scrol
   metrics.contentInset = RCTEdgeInsetsFromUIEdgeInsets(_scrollView.contentInset);
   metrics.containerSize = RCTSizeFromCGSize(_scrollView.bounds.size);
   metrics.zoomScale = _scrollView.zoomScale;
+
+  if (_layoutMetrics.layoutDirection == LayoutDirection::RightToLeft) {
+    metrics.contentOffset.x = metrics.contentSize.width - metrics.containerSize.width - metrics.contentOffset.x;
+  }
+
   return metrics;
 }
 
diff --git a/packages/react-native/React/Views/ScrollView/RCTScrollView.m b/packages/react-native/React/Views/ScrollView/RCTScrollView.m
index 6bd4b70898..16f300b976 100644
--- a/packages/react-native/React/Views/ScrollView/RCTScrollView.m
+++ b/packages/react-native/React/Views/ScrollView/RCTScrollView.m
@@ -1038,9 +1038,15 @@ RCT_SET_AND_PRESERVE_OFFSET(setScrollIndicatorInsets, scrollIndicatorInsets, UIE
     _coalescingKey++;
     _lastEmittedEventName = [eventName copy];
   }
+
+  CGPoint offset = scrollView.contentOffset;
+  if ([UIApplication sharedApplication].userInterfaceLayoutDirection == UIUserInterfaceLayoutDirectionRightToLeft) {
+    offset.x = scrollView.contentSize.width - scrollView.frame.size.width - offset.x;
+  }
+
   RCTScrollEvent *scrollEvent = [[RCTScrollEvent alloc] initWithEventName:eventName
                                                                  reactTag:self.reactTag
-                                                  scrollViewContentOffset:scrollView.contentOffset
+                                                  scrollViewContentOffset:offset
                                                    scrollViewContentInset:scrollView.contentInset
                                                     scrollViewContentSize:scrollView.contentSize
                                                           scrollViewFrame:scrollView.frame
diff --git a/packages/virtualized-lists/Lists/VirtualizedList.js b/packages/virtualized-lists/Lists/VirtualizedList.js
index e5ac5773d4..0306925242 100644
--- a/packages/virtualized-lists/Lists/VirtualizedList.js
+++ b/packages/virtualized-lists/Lists/VirtualizedList.js
@@ -1735,15 +1735,15 @@ class VirtualizedList extends StateSafePureComponent<Props, State> {
   _offsetFromScrollEvent(e: ScrollEvent): number {
     const {contentOffset, contentSize, layoutMeasurement} = e.nativeEvent;
     const {horizontal, rtl} = this._orientation();
-    if (Platform.OS === 'ios' || !(horizontal && rtl)) {
+    if (horizontal && rtl) {
+      return (
+        this._selectLength(contentSize) -
+        (this._selectOffset(contentOffset) +
+          this._selectLength(layoutMeasurement))
+      );
+    } else {
       return this._selectOffset(contentOffset);
     }
-
-    return (
-      this._selectLength(contentSize) -
-      (this._selectOffset(contentOffset) +
-        this._selectLength(layoutMeasurement))
-    );
   }
 
   _scheduleCellsToRenderUpdate(opts?: {allowImmediateExecution?: boolean}) {