The obvious MaxWidth gets ignored and the text in the "DisplayBox" TextBlock displays the whole text even if this text continues past the parent container controls (to the edge of the silverlight area.

<win:HierarchicalDataTemplate x:Key="hierarchicalTemplate" ItemsSource="{Binding _children}">
    <Border BorderThickness="0" BorderBrush="Orange" HorizontalAlignment="Stretch" Background="{Binding Converter={StaticResource BackgroundConverter}}">
        <toolkit:DockPanel LastChildFill="True" Width="{Binding HeirarchyLevel, Converter={StaticResource WidthConverter}}" Height="20">
            <Canvas toolkit:DockPanel.Dock="Right" Width="20" MouseLeftButtonUp="Arrow_MouseLeftButtonDown">
                <Rectangle Width="20" Height="20" Fill="Transparent" />
                <Line Stroke="Black" X1="5" Y1="10" X2="17" Y2="10" />
                <Line Stroke="Black" X1="11" Y1="5" X2="17" Y2="10" />
                <Line Stroke="Black" X1="11" Y1="15" X2="17" Y2="10" />
            </Canvas>
            <Ellipse Canvas.Top="5" Width="10" Height="10" Fill="Green" toolkit:DockPanel.Dock="Right" MouseLeftButtonDown="Ellipse_MouseLeftButtonDown" />
            <Canvas Width="Auto" Loaded="TextArea_Loaded">
                <TextBlock Name="DisplayBox" FontFamily="Arial" FontSize="17" Foreground="Black" Width="Auto" Text="{Binding TaskName}" MouseLeftButtonUp="TextBlock_MouseLeftButtonUp" /> 
                <TextBox Name="EditBox" FontFamily="Arial" FontSize="10" Foreground="Black" Height="20" Text="{Binding TaskName}" Visibility="Collapsed" LostFocus="TextBox_LostFocus" />
                <Line Stroke="Black" X1="0" Y1="10" X2="202" Y2="10" Width="Auto" />
            </Canvas>
        </toolkit:DockPanel>
    </Border>
</win:HierarchicalDataTemplate>