Fix TreeView missing children issue

dev/TreeView/TestUI/TreeViewPage.xaml

@@ -74,6 +74,7 @@
                     <Button x:Name="ResetItemsSource" AutomationProperties.Name="ResetItemsSource" Content="ResetItemsSource" HorizontalAlignment="Stretch" Margin="1" Click="ResetItemsSource_Click"/>
                     <Button x:Name="ResetItemsSourceAsync" AutomationProperties.Name="ResetItemsSourceAsync" Content="ResetItemsSourceAsync" HorizontalAlignment="Stretch" Margin="1" Click="ResetItemsSourceAsync_Click"/>
                     <Button x:Name="ExpandRootNode" AutomationProperties.Name="ExpandRootNode" Content="ExpandRootNode" HorizontalAlignment="Stretch" Margin="1" Click="ExpandRootNode_Click"/>
+                    <Button x:Name="SwapItemsSource" AutomationProperties.Name="SwapItemsSource" Content="SwapItemsSource" HorizontalAlignment="Stretch" Margin="1" Click="SwapItemsSource_Click"/>
                 </StackPanel>
             </ScrollViewer>
             <ScrollViewer Grid.Column="1">

dev/TreeView/TestUI/TreeViewPage.xaml.cs

@@ -998,5 +998,20 @@ private void ExpandRootNode_Click(object sender, RoutedEventArgs e)
         {
             TestTreeViewItemsSource[0].IsExpanded = !TestTreeViewItemsSource[0].IsExpanded;
         }
+
+        private bool isTreeView1SourceInUse = true;
+        private void SwapItemsSource_Click(object sender, RoutedEventArgs e)
+        {
+            if (!isTreeView1SourceInUse)
+            {
+                this.ContentModeTestTreeView.ItemsSource = TestTreeViewItemsSource;
+                isTreeView1SourceInUse = true;
+            }
+            else
+            {
+                this.ContentModeTestTreeView.ItemsSource = TestTreeView2ItemsSource;
+                isTreeView1SourceInUse = false;
+            }
+        }
     }
 }

dev/TreeView/TreeViewList.cpp

@@ -99,6 +99,14 @@ void TreeViewList::OnContainerContentChanging(const winrt::IInspectable& /*sende
     if (!args.InRecycleQueue())
     {
         auto targetItem = args.ItemContainer().as<winrt::TreeViewItem>();
+
+        // "Re-evaluate" the item source binding to trigger a TreeViewItem.OnPropertChanged(ItemsSourceProperty) changed call.
+        if (auto itemsSource = targetItem.ItemsSource())
+        {
+            targetItem.ItemsSource(nullptr);
+            targetItem.ItemsSource(itemsSource);
+        }     
+
         auto targetNode = NodeFromContainer(targetItem);
         auto treeViewItem = winrt::get_self<TreeViewItem>(targetItem);
         treeViewItem->UpdateIndentation(targetNode.Depth());