AvalonDock DocumentContent not garbage collected

I have created an application that makes use of the AvalonDock framework. A key part is the ability to edit domain-model entities using AvalonDock.DocumentContent derived editors. I hit upon a problem and discovered the my editors are not being garbage collected after they are closed and removed from the DockingManager.Documents collection.

After some fruitless searching I created a small test application that can be recreated in the following manner:

• In Visual Studio (I'm using 2008), create a new WPF application called AvalonDockLeak;
• Add a reference to the AvalonDock library (my version is 1.3.3571.0);
• Add a new UserControl called Document;

• Change Document.xmal to:

  <ad:DocumentContent x:Class="AvalonDockLeak.Document"
                      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                      xmlns:ad="clr-namespace:AvalonDock;assembly=AvalonDock">
      <Grid>
          <TextBox />
      </Grid>
  </ad:DocumentContent>
  

• Change Document.xmal.cs to:

  namespace AvalonDockLeak
  {
      using AvalonDock;
  
      public partial class Document : DocumentContent
      {
          public Document()
          {
              InitializeComponent();
          }
  
          ~Document()
          {
          }
      }
  }
  

  The destructor I have added to be able to diagnose the problem adding a breakpoint on the methods opening {, and seeing if it gets hit. It always does on closing the test application but not earlier.

• Now change Window1.xaml to:

  <Window x:Class="AvalonDockLeak.Window1"
          xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
          xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
          xmlns:ad="clr-namespace:AvalonDock;assembly=AvalonDock"
          Title="Memory Leak Test" Height="300" Width="300">
      <Grid>
          <Grid.RowDefinitions>
              <RowDefinition Height="Auto" />
              <RowDefinition />
          </Grid.RowDefinitions>
          <Button Name="NewButton" Click="NewButton_Click" Content="New" Height="26" Width="72" />
          <ad:DockingManager x:Name="DockManager" Grid.Row="1">
              <ad:DocumentPane />
          </ad:DockingManager>
      </Grid>
  </Window>
  

• Change Window1.xaml.cs to:

  namespace AvalonDockLeak
  {
      using System.Windows;
  
      public partial class Window1 : Window
      {
          private int counter = 0;
  
          public Window1()
          {
              InitializeComponent();
          }
  
          private void NewButton_Click(object sender, RoutedEventArgs e)
          {
              string name = "Document" + (++this.counter).ToString();
              var document = new Document()
              {
                  Name = name,
                  Title = name,
                  IsFloatingAllowed = false
              };
  
              document.Show(this.DockManager);
              document.Activate();
          }
      }
  }
  

This simple application also contains the leak. Which can be observed by the breakpoint on the ~Document() opening { not getting hit after closing a DocumentContent.

Now what I want to now is, is this a known problem and is there a way to prevent it? If the objects are only garbage collected after a long time then what can I do to expedite this? Calling GC.Collect() does not help by the way.