关于WPF中RichTextBox失去焦点后如何保持高亮显示所选择的内容

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="30"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <StackPanel Grid.Row="0" Orientation="Horizontal" FocusManager.IsFocusScope="True">
            <ComboBox Width="100">
                <ComboBoxItem>1</ComboBoxItem>
                <ComboBoxItem>2</ComboBoxItem>
                <ComboBoxItem>3</ComboBoxItem>
            </ComboBox>
            <ComboBox Width="100">
                <ComboBoxItem>a</ComboBoxItem>
                <ComboBoxItem>b</ComboBoxItem>
                <ComboBoxItem>c</ComboBoxItem>
            </ComboBox>
        </StackPanel>
        <RichTextBox Grid.Row="1"/>
    </Grid>
</Window>