The key part is: '<StackPanel Orientation="Horizontal" />'
It's this that controls the direction. of item display.
The next two parts give us the correct scrolling behaviour that we want.
Setting 'ScrollViewer.HorizontalScrollBarVisibility="Auto"' enables horizontal scrolling.
Setting 'ScrollViewer.VerticalScrollBarVisibility="Disabled"' prevents vertical scrolling.
The thing to note is that setting XxxxScrollBarVisibility doesn't just control the visibility if the scrollbar it also controls whether scrolling in that direction is possible.