Fix Horizontal ScrollView's scroll position during layout changes

Summary: Fix the calculation of offsetX in onLayout (ReactHorizontalScrollContainerView.java) that re-positions the updated layout. A private instance variable (oldWidth) is added in order to track the width difference between consecutive updates. (Issue report: #19979)

Reviewed By: mdvacca

Differential Revision: D8772780

fbshipit-source-id: 969dcead550f4a3d24d06416b63d960492b7a124

ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactHorizontalScrollContainerView.java

@@ -14,11 +14,13 @@
 public class ReactHorizontalScrollContainerView extends ViewGroup {
 
   private int mLayoutDirection;
+  private int mCurrentWidth;
 
   public ReactHorizontalScrollContainerView(Context context) {
     super(context);
     mLayoutDirection =
         I18nUtil.getInstance().isRTL(context) ? LAYOUT_DIRECTION_RTL : LAYOUT_DIRECTION_LTR;
+    mCurrentWidth = 0;
   }
 
   @Override
@@ -32,12 +34,12 @@ protected void onLayout(boolean changed, int left, int top, int right, int botto
       setLeft(newLeft);
       setRight(newRight);
 
-      // Fix the ScrollX position when using RTL language
-      int offsetX = computeHorizontalScrollRange() - getScrollX();
-
       // Call with the present values in order to re-layout if necessary
       HorizontalScrollView parent = (HorizontalScrollView) getParent();
+      // Fix the ScrollX position when using RTL language
+      int offsetX = parent.getScrollX() + getWidth() - mCurrentWidth;
       parent.scrollTo(offsetX, parent.getScrollY());
     }
+    mCurrentWidth = getWidth();
   }
 }