fix(reg): Fix SelectedIndex update when ItemsSource is ObservableColl…

…ection

Fix regression introduced because we were 'double dipping' the collection changed event, both directly on the ItemsSource and via ItemsControl.Items.

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_ListViewBase_Items.cs

@@ -413,6 +413,33 @@ public void When_ItemsSource_Grouped_Observables_Outer_Modified()
 #endif
 		}
 
+		[TestMethod]
+		public void When_ItemsSource_ObservableCollection_Selection()
+		{
+			var itemsSource = new ObservableCollection<string>(Enumerable.Range(0, 80).Select(i => $"Item {i}"));
+			var SUT = new ListView { ItemsSource = itemsSource };
+
+			SUT.SelectedIndex = 5;
+
+			Assert.AreEqual(5, SUT.SelectedIndex);
+			Assert.AreEqual("Item 5", SUT.SelectedItem);
+
+			itemsSource.RemoveAt(2);
+
+			Assert.AreEqual(4, SUT.SelectedIndex);
+			Assert.AreEqual("Item 5", SUT.SelectedItem);
+
+			itemsSource.RemoveAt(22);
+
+			Assert.AreEqual(4, SUT.SelectedIndex);
+			Assert.AreEqual("Item 5", SUT.SelectedItem);
+
+			itemsSource.Insert(1, "Item 1.5");
+
+			Assert.AreEqual(5, SUT.SelectedIndex);
+			Assert.AreEqual("Item 5", SUT.SelectedItem);
+		}
+
 		private static ObservableCollection<GroupingObservableCollection<TKey, TElement>> GetGroupedObservable<TKey, TElement>(IEnumerable<TElement> source, Func<TElement, TKey> keySelector)
 		{
 			var observables = source.GroupBy(keySelector).Select(g => new GroupingObservableCollection<TKey, TElement>(g.Key, g));

src/Uno.UI/UI/Xaml/Controls/Primitives/Selector.cs

@@ -507,10 +507,14 @@ internal virtual void OnItemClicked(int clickedIndex) { }
 
 		protected override void OnItemsChanged(object e)
 		{
-			if (e is IVectorChangedEventArgs iVCE)
+			if (
+				// When ItemsSource is set, we get collection changes from it directly (and it's not possible to directly modify Items)
+				ItemsSource == null &&
+				e is IVectorChangedEventArgs iVCE
+			)
 			{
 				if (iVCE.CollectionChange == CollectionChange.ItemChanged
-				    || (iVCE.CollectionChange == CollectionChange.ItemInserted && iVCE.Index < Items.Count))
+					|| (iVCE.CollectionChange == CollectionChange.ItemInserted && iVCE.Index < Items.Count))
 				{
 					var item = Items[(int)iVCE.Index];
 
@@ -624,7 +628,7 @@ protected void SetFocusedItem(int index,
 									  FocusState focusState,
 									  bool animateIfBringIntoView)
 		{
-			
+
 		}
 
 		protected void SetFocusedItem(int index,
@@ -711,7 +715,7 @@ protected void SetFocusedItem(int index,
 
 		private void ScrollIntoView(int index, bool isGroupItemIndex, bool isHeader, bool isFooter, bool isFromPublicAPI, bool ensureContainerRealized, bool animateIfBringIntoView, ScrollIntoViewAlignment @default)
 		{
-			
+
 		}
 
 		protected void SetFocusedItem(int index,
@@ -748,7 +752,7 @@ protected void SetFocusedItem(int index,
 		protected void SetFocusedItem(int index,
 									  bool shouldScrollIntoView)
 		{
-			
+
 		}
 
 		bool CanScrollIntoView()