I can't check right now (ISP still being incompetent; at the library), but I think the ScrollXLeft, ScrollYTop, ScrollXRight, ScrollYBottom expressions give the current viewport, and if you add a layer parameter, it should give the appropriate values for that layer. Eg. on a parallaxed layer "Stars", ScrollXLeft("Stars") should give the X co-ordinate of the left visible edge. If this isn't implemented, then I'll try get it in to the next build. You could then use these values to determine if the stars have left the viewport of the parallaxed layer.