Firing UIScrollView's scrollViewDidScroll: delegate call from within …

…Xaml ScrollViewer's ScrollViewerViewChanged event leads to an AV in Xaml if the delegate removes the UIScrollView from the Xaml UIElement tree. To work around the issue, push the delegate call to a subsequent run of the UI thread.

Fixes microsoft#1865.

Frameworks/UIKit/UIScrollView.mm

@@ -894,7 +894,11 @@ - (void)_refreshOrigin {
     [self setNeedsLayout];
 
     if ([self.delegate respondsToSelector:@selector(scrollViewDidScroll:)]) {
-        [self.delegate scrollViewDidScroll:self];
+        // NOTE: We have to push this off of the ScrollViewer's callback thread to work around
+        // issue #1865.
+        dispatch_async(dispatch_get_main_queue(), ^{
+            [self.delegate scrollViewDidScroll:self];
+        });
     }
 }