Active Files button on Tabbed MDI container is disabled

I have created a simple demo of my problem (see below). Each time the Add button is clicked (top left) a document is added to the tabbed MDI container. The problem is that the Active Files dropdown button at the top right of the tabbed MDI container remains disabled as I add documents. Strangely, it becomes enabled when I close any of the documents, and remains enabled thereafter. Is this a bug, or am I doing something wrong?

Peter

Here is the XAML for the main window

<Window x:Class="ActiproDockingTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
		xmlns:apd="http://schemas.actiprosoftware.com/winfx/xaml/docking"
        Title="MainWindow" Height="350" Width="525">
	<DockPanel>
		<Button DockPanel.Dock="Top" Content="Add" Click="Add_Click" HorizontalAlignment="Left"/>
		<apd:DockSite>
			<apd:Workspace>
				<apd:TabbedMdiHost>
					<apd:TabbedMdiContainer x:Name="mdiTab"/>
				</apd:TabbedMdiHost>
			</apd:Workspace>
		</apd:DockSite>
	</DockPanel>
</Window>

 And here is the C#

using System.Windows;
using System.Windows.Controls;
using ActiproSoftware.Windows.Controls.Docking;

namespace ActiproDockingTest
{
	public partial class MainWindow : Window
	{
		public MainWindow()
		{
			InitializeComponent();
		}

		int count = 0;

		private void Add_Click(object sender, RoutedEventArgs e)
		{
			int i = ++count;
			AddTab(new TextBlock() { Text = "This is document " + i }, "Document " + i);
		}

		private void AddTab(object tabContent, string title)
		{
			DockingWindow window = new DockingWindow();
			window.Content = tabContent;
			window.Title = title;
			mdiTab.Items.Add(window);
			window.Activate();
		}
	}
}