unbounded world

By default in Greenfoot, the World class prevents any movement outside the bounds of the world. So if you try setLocation(-30, -10), it won't work (I think it silently fails?). The unbounded world relaxes this restriction, so that things can be "in" the world, but not in the visible portion in the Greenfoot IDE. It's mainly useful for doing things like scrolling shoot 'em-ups, or in things like physics simulations where you want an orbiting entity to have motion that takes it out of the world and back in again.

thanks. i follow that. what i am looking for is a way for the "viewport" to be moveable around the larger world. for example, if there are stationary actors that are outside the viewport, can they be brought into view, not by navigating them back into view, but by sliding the viewport vertically or horizontally to discover them? btw, is discussion traffic not pushed to the subscriber's email inbox?

I'm afraid greenfoot has no conception of a viewport. There is the world, which is displayed from 0,0 to width, height. If you want to scroll around a large world, you would need to do so by moving all the actors in the world (which is only four lines of code: get a list from the world and call setLocation on them).