In Shoes, any container can have a border. The border draws along the outside of the container, either for visual appeal or for debugging purposes. It can be a good visual cue as well. When writing Shoes code, I often give all my containers a red border so I have a visual identifier of where they are. When the application is finished, the red borders are removed.
Shoes.app :width => 300, :height => 150 do
:stroke => steelblue