Work around dotnet/wpf#122 in Document Outline

src/VisualStudio/Core/Def/DocumentOutline/DocumentOutlineView.xaml

@@ -77,22 +77,16 @@
         </Grid.RowDefinitions>
         <ContentControl Grid.Row="0" Name="Commands" Focusable="True" />
         <Grid Grid.Row="1" x:Name="SearchHost" />
-        <!-- Two important properties are being set for TreeView:
-               We set IsVirtualizing to "True" so that WPF only generates internal data-structures elements that are visible.
-               Setting VirtualizationMode to "Recycling" ensures that WPF internal data is reused as items scroll in and out of view.
-        -->
         <!-- Note: we register TreeViewItem.SourceUpdated and set `NotifyOnSourceUpdated=True, NotifyOnTargetUpdated=False`
              for the item `IsSelected` event.  This is important for a reentrancy issue we would otherwise have.  Specifically,
              As the user moves around, we want to expand/select the correct item of interest in the tree.  However, we do not 
              want that *programmatic* selection to then end up firing an event saying "i was selected" which we then interpret
              to mean that we should navigate to that item.  By only hearing when the source is updated (meaning the user actually
              clicked on the tree view item, not that we programmatically set it) we only actual perform the navigation on user
              interaction. -->
-        <TreeView Grid.Row="2"
-                  Name="SymbolTree"
+        <self:VirtualizingTreeView Grid.Row="2"
+                  x:Name="SymbolTree"
                   AutomationProperties.Name="{x:Static self:DocumentOutlineStrings.Document_Outline}"
-                  VirtualizingStackPanel.IsVirtualizing="True"
-                  VirtualizingStackPanel.VirtualizationMode="Recycling"
                   SourceUpdated="SymbolTree_SourceUpdated"
                   TreeViewItem.Selected="SymbolTreeItem_Selected"
                   Visibility="{Binding Visibility, Mode=OneWayToSource}"
@@ -130,6 +124,6 @@
                     <Setter Property="Margin" Value="0, 0, 0, 1" />
                 </Style>
             </TreeView.ItemContainerStyle>
-        </TreeView>
+        </self:VirtualizingTreeView>
     </Grid>
 </UserControl>

src/VisualStudio/Core/Def/DocumentOutline/VirtualizingTreeView.cs

@@ -0,0 +1,49 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Windows.Automation.Peers;
+using System.Windows.Controls;
+
+namespace Microsoft.VisualStudio.LanguageServices.DocumentOutline;
+
+// Provide a workaround for https://github.com/dotnet/wpf/issues/122 when using virtualized TreeView.
+internal sealed class VirtualizingTreeView : TreeView
+{
+    public VirtualizingTreeView()
+    {
+        // Two important properties are being set for TreeView:
+        // We set IsVirtualizing to "true" so that WPF only generates internal data-structures elements that are visible.
+        // Setting VirtualizationMode to "Recycling" ensures that WPF internal data is reused as items scroll in and out of view.
+        VirtualizingPanel.SetIsVirtualizing(this, true);
+        VirtualizingPanel.SetVirtualizationMode(this, VirtualizationMode.Recycling);
+    }
+
+    protected override AutomationPeer OnCreateAutomationPeer()
+        => new VirtualizingTreeViewAutomationPeer(this);
+
+    public sealed class VirtualizingTreeViewAutomationPeer(TreeView owner)
+        : TreeViewAutomationPeer(owner)
+    {
+        protected override ItemAutomationPeer CreateItemAutomationPeer(object item)
+            => new VirtualizingTreeViewDataItemAutomationPeer(item, this, null);
+    }
+
+    public sealed class VirtualizingTreeViewDataItemAutomationPeer(object item, ItemsControlAutomationPeer itemsControlAutomationPeer, TreeViewDataItemAutomationPeer? parentDataItemAutomationPeer)
+        : TreeViewDataItemAutomationPeer(item, itemsControlAutomationPeer, parentDataItemAutomationPeer)
+    {
+        protected override string GetNameCore()
+        {
+            try
+            {
+                return base.GetNameCore();
+            }
+            catch (NullReferenceException)
+            {
+                // https://github.com/dotnet/wpf/issues/122
+                return "";
+            }
+        }
+    }
+}