I saw Tim's challenge and thought about some interesting single element XAML. One of my markup which I thought will cover the whole screen does not do that anymore, it just covers about 80% of the screen MAX. I remember going thru the threat modeling and having Markup injection like this a concern. The following markup definitely covers the whole screen 12 months ago - and my application is running full-trust.

<Popup IsOpen="true" StaysOpen="true" Placement="Absolute" HorizontalOffset="0" VerticalOffset="0" 
xmlns= "
http://schemas.microsoft.com/winfx/2006/xaml/presentation" Width="2000" Height="2000"/>

This is all good, just signs that WPF is wrapping up and ready to ship :-).