Resize applic. or splitcontainer does not fire sizechanged

<Window x:Class="Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:docking="http://schemas.actiprosoftware.com/winfx/xaml/docking"
    xmlns:ribbon="http://schemas.actiprosoftware.com/winfx/xaml/ribbon"
    xmlns:shared="http://schemas.actiprosoftware.com/winfx/xaml/shared"
        xmlns:loc="clr-namespace:WpfApplication1"

    Title="Window1" Height="300" Width="450">
    <DockPanel>
        <docking:DockSite x:Name="MainDockSite" >
            <docking:Workspace  HorizontalAlignment="Stretch" >
                <docking:TabbedMdiHost Name="MDIPresentationsWindow" >
                    <docking:TabbedMdiContainer Name="MDITabContainer" >
                        <docking:DocumentWindow Name="DocWIndowPage1" Title="Page1">
                            <!--<loc:UserControl1 x:Name="AA"></loc:UserControl1>-->
                        </docking:DocumentWindow>
                    </docking:TabbedMdiContainer>
                </docking:TabbedMdiHost>
            </docking:Workspace>
        </docking:DockSite>
    </DockPanel>
</Window>

Imports ActiproSoftware.Windows.Controls.Docking

Class Window1

  Public Sub New()

    ' This call is required by the Windows Form Designer.
    InitializeComponent()

  End Sub


  Private Sub DocWIndowPage1_SizeChanged(ByVal sender As Object, ByVal e As System.Windows.SizeChangedEventArgs) Handles DocWIndowPage1.SizeChanged
    Dim b As Boolean = True 'is only fired once....
  End Sub
End Class