FluentRibbon bug

<fluent:RibbonWindow x:Class="WpfApplication15.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication15"
        xmlns:fluent="urn:fluent-ribbon"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <fluent:RibbonWindow.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <!-- Because of threading issues the metro style is now only applied to the mainwindow. Root cause: Task.Factory => Thread with RecipientDialog -->
                <ResourceDictionary Source="pack://application:,,,/fluent;Component/Themes/Office2013/Generic.xaml" />

            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </fluent:RibbonWindow.Resources>

    <Grid>
        <fluent:Ribbon x:Name="Ribbon" AutomaticStateManagement="False" 
									   IsQuickAccessToolBarVisible="False" 
									   CanCustomizeQuickAccessToolBar="False" 
									   CanCustomizeRibbon="False"
                                       CanMinimize="False"
									   CanQuickAccessLocationChanging="False" BorderThickness="3" 
									>

            <fluent:RibbonTabItem x:Name="homeTabItem"
                                  Header="Home"
                                  KeyTip="H"
                                  ToolTip="Regular Tool Tip">
                <fluent:RibbonGroupBox 
                                       KeyTip="T10"
                                       x:Name="C"
                                       Header="Themes (Office 2010)"
                                       CanAddToQuickAccessToolBar="False"
                                       IsLauncherVisible="False">
                    <fluent:Button 
                                  
                                   SizeDefinition="Middle">Silver</fluent:Button>
                    <fluent:Button 
                                 
                                   SizeDefinition="Middle">Black</fluent:Button>
                    <fluent:Button 
                                 
                                   SizeDefinition="Middle">Blue</fluent:Button>
                    <fluent:CheckBox Name="DontUseDwm"
                                    >Don't use DWM</fluent:CheckBox>
                </fluent:RibbonGroupBox>

            
            </fluent:RibbonTabItem>

        </fluent:Ribbon>
    </Grid>
</fluent:RibbonWindow>