Bottom of window missing in .NET 4.0

<ribbon:RibbonWindow x:Class="ActiproRibbonWindowTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:ribbon="http://schemas.actiprosoftware.com/winfx/xaml/ribbon"
        xmlns:docking="http://schemas.actiprosoftware.com/winfx/xaml/docking"
        Title="MainWindow" Height="350" Width="525">
    <docking:DockSite 
        x:Name="dockSite"
        >
        <docking:SplitContainer>
            <docking:SplitContainer Orientation="Vertical">

                <!-- Workspace -->
                <docking:Workspace>
                    <docking:TabbedMdiHost>
                        <docking:TabbedMdiContainer>
                            <docking:DocumentWindow Title="Document1.txt">
                                <TextBox Text="Where did Tool Window 2 &amp; 4 go?" />
                            </docking:DocumentWindow>
                        </docking:TabbedMdiContainer>
                    </docking:TabbedMdiHost>
                </docking:Workspace>

                <docking:ToolWindowContainer>
                    <docking:ToolWindow Title="Tool Window 3" />
                    <docking:ToolWindow Title="Tool Window 4" />
                </docking:ToolWindowContainer>

            </docking:SplitContainer>

            <docking:ToolWindowContainer>
                <docking:ToolWindow Title="Tool Window 1" />
                <docking:ToolWindow Title="Tool Window 2" />
            </docking:ToolWindowContainer>

        </docking:SplitContainer>
    </docking:DockSite>
</ribbon:RibbonWindow>