Restore customization patch of VirtualizedList which inverts mouse wh…

…eel scroll position while list is inverted. Added loud comments to make it clear this code intends to live on in RNW even though not present in RN.

packages/react-native-web/src/vendor/react-native/VirtualizedList/index.js

@@ -680,6 +680,9 @@ class VirtualizedList extends React.PureComponent<Props, State> {
 
   state: State;
 
+  // REACT-NATIVE-WEB patch to preserve during future RN merges: Support inverted wheel scroller.
+  invertedWheelEventHandler: ?(ev: any) => void;
+
   constructor(props: Props) {
     super(props);
     invariant(
@@ -733,6 +736,20 @@ class VirtualizedList extends React.PureComponent<Props, State> {
       }
     }
 
+    // REACT-NATIVE-WEB patch to preserve during future RN merges: Support inverted wheel scroller.
+    // For issue https://github.com/necolas/react-native-web/issues/995
+    this.invertedWheelEventHandler = (ev: any) => {
+      if (this.props.inverted && this._scrollRef && this._scrollRef.getScrollableNode) {
+        const node = (this._scrollRef: any).getScrollableNode();
+        if (this.props.horizontal) {
+          node.scrollLeft -= ev.deltaX || ev.wheelDeltaX
+        } else {
+          node.scrollTop -= ev.deltaY || ev.wheelDeltaY
+        }
+        ev.preventDefault();
+      }
+    };
+
     this.state = initialState;
   }
 
@@ -749,6 +766,9 @@ class VirtualizedList extends React.PureComponent<Props, State> {
         parentDebugInfo: this.context.debugInfo,
       });
     }
+
+    // REACT-NATIVE-WEB patch to preserve during future RN merges: Support inverted wheel scroller.
+    this.setupWebWheelHandler();
   }
 
   componentWillUnmount() {
@@ -768,6 +788,30 @@ class VirtualizedList extends React.PureComponent<Props, State> {
       tuple.viewabilityHelper.dispose();
     });
     this._fillRateHelper.deactivateAndFlush();
+
+    // REACT-NATIVE-WEB patch to preserve during future RN merges: Support inverted wheel scroller.
+    this.teardownWebWheelHandler();
+  }
+
+  // REACT-NATIVE-WEB patch to preserve during future RN merges: Support inverted wheel scroller.
+  setupWebWheelHandler() {
+    if (this._scrollRef && this._scrollRef.getScrollableNode) {
+      this._scrollRef.getScrollableNode().addEventListener('wheel',
+          this.invertedWheelEventHandler
+      );
+    } else {
+      setTimeout(() => this.setupWebWheelHandler(), 50);
+      return
+    }
+  }
+
+  // REACT-NATIVE-WEB patch to preserve during future RN merges: Support inverted wheel scroller.
+  teardownWebWheelHandler() {
+    if (this._scrollRef && this._scrollRef.getScrollableNode) {
+      this._scrollRef.getScrollableNode().removeEventListener('wheel',
+          this.invertedWheelEventHandler
+      );
+    }
   }
 
   static getDerivedStateFromProps(newProps: Props, prevState: State): State {