Silverlight Border FAIL !!

[Complete]

Silverlight Border FAIL !!
I have a border in XAML code that is defined right after that main canvas:

<Canvas xmlns="[url=http://schemas.microsoft.com/client/2007]Error[/url]"
...      VerticalAlignment="Top" HorizontalAlignment="Center">
    <Border x:Name="JohnnyBorder" BorderThickness="1" Margin="0,0,0,0" BorderBrush="#FF677B8B" VerticalAlignment="Stretch"
            HorizontalAlignment="Center">

And in the code I position the border ike this:

 JohnnyBorder = (Border)appCanvas.FindName("JohnnyBorder");
                    JohnnyBorder.Height = App.Current.Host.Content.ActualHeight;
                    JohnnyBorder.Width = App.Current.Host.Content.ActualWidth;

But it does not work. JohnnyBoarder's bottom line is a few percentages above the bottom of the browser. Why?