Added failing test for #3328.

tests/Avalonia.Controls.UnitTests/TreeViewTests.cs

@@ -971,6 +971,40 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Fact]
+        public void Removing_TreeView_From_Root_Should_Preserve_TreeViewItems()
+        {
+            // Issue #3328
+            var tree = CreateTestTreeData();
+            var target = new TreeView
+            {
+                Template = CreateTreeViewTemplate(),
+                Items = tree,
+            };
+
+            var root = new TestRoot();
+            root.Child = target;
+
+            CreateNodeDataTemplate(target);
+            ApplyTemplates(target);
+            ExpandAll(target);
+
+            Assert.Equal(5, target.ItemContainerGenerator.Index.Containers.Count());
+
+            root.Child = null;
+
+            Assert.Equal(5, target.ItemContainerGenerator.Index.Containers.Count());
+            Assert.Equal(1, target.Presenter.Panel.Children.Count);
+
+            var rootNode = Assert.IsType<TreeViewItem>(target.Presenter.Panel.Children[0]);
+            Assert.Equal(3, rootNode.ItemContainerGenerator.Containers.Count());
+            Assert.Equal(3, rootNode.Presenter.Panel.Children.Count);
+
+            var child2Node = Assert.IsType<TreeViewItem>(rootNode.Presenter.Panel.Children[1]);
+            Assert.Equal(1, child2Node.ItemContainerGenerator.Containers.Count());
+            Assert.Equal(1, child2Node.Presenter.Panel.Children.Count);
+        }
+
         private void ApplyTemplates(TreeView tree)
         {
             tree.ApplyTemplate();