I could be interesting to compare that to the Android/iOS/HTML5 equivalent.

Basically this XAML does 3 things:

- declare 2 states: narrowState for windows smaller than 640px widestate for windows wider >= 641px

- set the layout stacking strategy of a container to Vertical when in narrowState

- set a top margin of a button to 4px when in narrowState (not sure this is relevant for this sample)