Send custom events when the structureboard is shown and hidden.

This allows websites to adapt by changing page styles for example, so that
all placeholders are visible. This is useful for rotators and other features
that hide parts of the page or place them offscreen.
Possibly, but $(window) is more efficient because the event doesn't bubble anywhere from $(window). Whereas if it's sent to $(document), then it bubbles up to $(window). So in fact you could attach listeners to either object and they would work. That seems confusing to me. If the event is fired at $(window) then that's the only place you can to listen for it.