DropShadowPanel streched in a Viewbox

Hi Guys,

If I put a DropShadowPanel inside of a Viewbox with Strech="Fill" the DropShadowPanel doesn't work like expected here.

Take a look on my Screenshots for this:

You can use the following SourceCode to check this:

<Viewbox Stretch="Fill" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Margin="100,100,100,100">
            <Grid>
                <extControls:DropShadowPanel ShadowOpacity="1"
                                  BlurRadius="10"
                                  Color="Black"
                                  x:Name="MyShadow">
                    <Rectangle x:Name="Rect" Fill="Red" Width="100" Height="100" />
                </extControls:DropShadowPanel>
            </Grid>
        </Viewbox>

Best Regards Thilo