.NET 4.0 TextOptions.TextFormattingMode truncates tab labels

<ribbon:RibbonWindow x:Class="WpfApplication1.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"
                     
        TextOptions.TextFormattingMode="Display"
                     
        Height="600" Width="800">
    <DockPanel>
        
        <ribbon:Ribbon DockPanel.Dock="Top">
            
            <ribbon:Ribbon.Tabs>
                
                <ribbon:Tab Label="Home" />
                <ribbon:Tab Label="Validation" />
                <ribbon:Tab Label="Storage" />
                
            </ribbon:Ribbon.Tabs>
            
        </ribbon:Ribbon>
        
        <Label>
            Notice how the longer tabs are cut off.
        </Label>
        
    </DockPanel>
</ribbon:RibbonWindow>

<ribbon:RibbonWindow x:Class="Pallas.GUI.TestWindow"
    xmlns="http://schemas.microsoft.com/netfx/2007/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:ribbon="http://schemas.actiprosoftware.com/winfx/xaml/ribbon"
    Title="TestWindow">
    <DockPanel LastChildFill="True">
        <ribbon:Ribbon DockPanel.Dock="Top">
            <ribbon:Tab Label="Algemeen"/>
            <ribbon:Tab Label="A very, very long tab title"/>
        </ribbon:Ribbon>
    </DockPanel>
</ribbon:RibbonWindow>

public TestWindow()
{
  Style defaultTextBlockStyle = new Style() { TargetType = typeof(TextBlock) };
  defaultTextBlockStyle.Setters.Add(new Setter(TextBlock.FontSizeProperty, (double)20));
  Application.Current.Resources.Add(typeof(TextBlock), defaultTextBlockStyle);

  InitializeComponent();
}

Typeface typeface = new Typeface(this.LabelControl.FontFamily, this.LabelControl.FontStyle, this.LabelControl.FontWeight, this.LabelControl.FontStretch);
FormattedText text = new FormattedText(this.Label, CultureInfo.CurrentCulture, this.LabelControl.FlowDirection, typeface, this.LabelControl.FontSize, Brushes.Black);