Why does the RadNavigationView show the PART_DismissOverlay hidden rectangle?

Question

I have a WPF App and I am using some Telerik controls, namely the RadNavigationView. My XAML is straightforward - shown below. What is bugging me is I have an area of the RadNavigationView that when looked at through the "Snoop" app, is always visible and is ugly as **@*. Would love to get rid of it. I can't believe this is stock behaviour for the nav view??

I have attached a few screen shots. Undersnoop it is a rectangle called "PART_DismissOverlay".

Compact Menu: (https://i.stack.imgur.com/vQFCa.png) Expanded Menu: (https://i.stack.imgur.com/OyqfM.png)

    <DockPanel>
        <!-- Top Panel -->
        <StackPanel DockPanel.Dock="Top" Orientation="Horizontal" Background="LightGray">
            <Label Content="{Binding CurrentDateTime, StringFormat='{}{0:dd/MM/yyyy HH:mm:ss}'}" Margin="5" />
        </StackPanel>

        <Grid DockPanel.Dock="Bottom">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto"/>
                <ColumnDefinition Width="*"/>
            </Grid.ColumnDefinitions>

            <!-- Navigation Panel -->
            <telerik:RadNavigationView x:Name="navigationView"
                           IsPaneOpen="True"
                           CompactModeThresholdWidth="10000"
                           ItemsSource="{Binding NavigationItems}"
                           SelectedItem="{Binding SelectedNavigationItem}"
                           ItemContainerStyle="{DynamicResource NavigationViewItemStyle}" Margin="0,0,0,0">

                <telerik:RadNavigationView.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Orientation="Horizontal" Margin="0,0,0,0">
                            <telerik:RadGlyph Glyph="{Binding IconGlyph}" />
                            <TextBlock Text="{Binding Title}" Margin="10,0,0,0" />
                        </StackPanel>
                    </DataTemplate>
                </telerik:RadNavigationView.ItemTemplate>

            </telerik:RadNavigationView>
            <ContentControl Grid.Column="1" Content="{Binding SelectedItem.ViewModel, ElementName=navigationView}"
                        ContentTemplateSelector="{StaticResource ViewModelDataTemplateSelector}" />
        </Grid>
    </DockPanel>

I tried setting the Margins and Padding to "0"; that did not work. I am trying to ensure that the Menu lines up with the Content Area border to the right. When you hover over the menu items the highlight only goes as far as the "PART_DismissOverlay" and well, it kinda looks ugly.