I'm trying to make a TextBox template and I need to include a ScrollViewer in the template - basically I want to add some content (like line numbers) that needs to scroll along with the normal text. The default template for the TextBox is like this:

                            <Border x:Name="Border" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" CornerRadius="1" Opacity="1">
                            <Grid>
                                <Border x:Name="ReadOnlyVisualElement" Background="#5EC9C9C9" Opacity="0"/>
                                <Border x:Name="MouseOverBorder" BorderBrush="Transparent" BorderThickness="1">
                                    <ScrollViewer x:Name="ContentElement" BorderThickness="0" IsTabStop="False" Padding="{TemplateBinding Padding}"/>
                                </Border>
                            </Grid>
                        </Border>

If I change the ContentElement from ScrollViewer to Border, for example, the TextBox behaves normally - i just lose the scrolling ability. Now, if I wrap the ContentElement with a ScrollViewer, it no longer displays the caret and selection - if you type, it still gets updated though.

                            <Border x:Name="Border" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" CornerRadius="1" Opacity="1">
                            <Grid>
                                <Border x:Name="ReadOnlyVisualElement" Background="#5EC9C9C9" Opacity="0"/>
                                <Border x:Name="MouseOverBorder" BorderBrush="Transparent" BorderThickness="1">
                                    <ScrollViewer ScrollViewer.HorizontalScrollBarVisibility="Auto" ScrollViewer.VerticalScrollBarVisibility="Auto" >
                                        <Border x:Name="ContentElement" BorderThickness="0" Padding="{TemplateBinding Padding}" />
                                    </ScrollViewer>
                                </Border>
                            </Grid>
                        </Border>

Any idea why this happens and how can I fix it?