What do I have to do to get the inner TextBlock below to wrap its text without defining an absolute width?

I've tried Width=Auto, Stretch, TextWrapping, putting it in a StackPanel, nothing seems to work.

XAML:

<UserControl x:Class="Test5.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:tk="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Toolkit"
    Width="800"
    Height="600">
    <tk:DockPanel LastChildFill="True">

        <StackPanel tk:DockPanel.Dock="Top"
            Width="Auto"
            Height="50"
            Background="#eee">
            <TextBlock Text="{Binding TopContent}"/>
        </StackPanel>

        <StackPanel tk:DockPanel.Dock="Bottom" Background="#bbb"
            Width="Auto"
            Height="50">
            <TextBlock Text="bottom area"/>
        </StackPanel>

        <StackPanel tk:DockPanel.Dock="Right" Background="#ccc"
            Width="200"
            Height="Auto">
            <TextBlock Text="info panel"/>
        </StackPanel>

        <StackPanel tk:DockPanel.Dock="Left" Background="#ddd"
            Width="Auto"
            Height="Auto">
            <ScrollViewer HorizontalScrollBarVisibility="Auto" Padding="10"
            BorderThickness="0"
                Width="Auto"
                VerticalScrollBarVisibility="Auto">
                <tk:DockPanel HorizontalAlignment="Left" Width="Auto" >
                    <StackPanel tk:DockPanel.Dock="Top" HorizontalAlignment="Left">
                        <Button Content="Add More" Click="Button_Click"/> 
                    </StackPanel>
                    <TextBlock tk:DockPanel.Dock="Top" 
                        Text="{Binding MainContent}" 
                        Width="Auto" 
                        TextWrapping="Wrap" />
                </tk:DockPanel>
            </ScrollViewer>
        </StackPanel>

    </tk:DockPanel>
</UserControl>