When selecting objects within a JgoView if I move the mouse outside of the window and release it, none of the objects within the selection rectangle are selected.
This is JGo standard behavior. If the mouse button is released outside of the JGoView, the internal mouse state is simply reset. You can change this behavior by overriding JGoView.onDragDropEnd() if you wish, but I would advise against it. Handling the low-level mouse events and internal JGo mouse state can be complex and could have unanticipated side effects.
If you don’t need to support dragging JGoObjects to other components/windows, you could try calling JGoView.setDragEnabled(false).