Two-Way Bindable TreeView.SelectedItem Behavior

 // check for top level item
 this.AssociatedObject.ExpandAll();
 this.AssociatedObject.UpdateLayout();

 are you sure that all containers will be loaded?
Try your sample with this source

treeNodes = new ObservableCollection<TreeNode>()
            {
                new TreeNode("First",
                    new TreeNode("Second"),
                    new TreeNode("Third"),
                    new TreeNode("Fourth", 
                        new TreeNode("9"),
                        ...
                        new TreeNode("16",
                            new TreeNode("17"),
                            ...
                            new TreeNode("29",
                                new TreeNode("30"),
                                ...
                                new TreeNode("36"))))
                        ),
                new TreeNode("Fifth",
                    new TreeNode("Sixth"),
                    new TreeNode("Sevent"),
                    new TreeNode("Eight"))
            }

and try to select 34 node in first treeView