Displaying items horizontaly in WPF ItemsControl

I am trying my hand on Microsoft Composite Application for few days. I was trying to display list of Actions available from a moudle into a Region using following markup:

This displays items in vertical manner (as is default with ItemsControl and its derivatives like ListBox). I added following code to make it use StackPanel:

But it did not produce the desired outcome. I tried the WrapPanel also but it was still same. After some more frustration I commented GroupStyle tag:

Now it worked as desired by displaying Button controls in a single row.