It would be nice to have a simple flag or set of flags to tell whether a
subcomponent in a ScrollPane was (a) fully visible, (b) partially obscured,
or (c) completely hidden because it is scrolled off screen. This is just a
function of the component's location() and the pane's getScrollPosition()
and getViewportSize(), but I can see it being called a lot and it would be
nice to have it built into the ScrollPane class itself.