Merge pull request #90 from erri120/fix/89

Fix incrementing `-1` indices

src/ObservableCollections.R3/ObservableCollectionR3Extensions.cs

@@ -167,10 +167,12 @@ protected override void Handler(in NotifyCollectionChangedEventArgs<T> eventArgs
                             eventArgs.Action,
                             item,
                             eventArgs.OldItem,
-                            i++,
+                            newStartingIndex: i,
                             eventArgs.OldStartingIndex,
                             eventArgs.SortOperation);
 
+                        if (eventArgs.NewStartingIndex != -1) i++;
+
                         observer.OnNext(newArgs);
                     }
                 }