I’m not sure if this is a bug or a feature.
When using a padding on a WrapPanel, the content seems to ignore the padding. As you can see from this example https://dl.dropboxusercontent.com/s/gfnnylcou3mxajj/shot_160408_130957.jpg
Both the WrapPanel and the Stack one have the same padding.
<App>
<WrapPanel>
<WrapPanel Padding="20" Background="#333" Margin="10">
<Text Value="WrapPanel" FontSize="23" Background="#ddd" />
</WrapPanel>
<StackPanel Padding="20" Background="#333" Margin="10" Alignment="Default" >
<Text Value="StackPanel" FontSize="23" Background="#ddd" />
</StackPanel>
</WrapPanel>
</App>