When you do SetViewport you're not really using the viewport to crop the graphic - a viewport is a definition of an area of the screen and how its coordinates translate to the world coordinates of objects. It is possible for drawing to occur outside of the viewport using the same scaling of coordinates. To stop that you have to use a `scissor rectangle` test so that it prevents drawing of pixels outside the viewport area. BRL has confused the term viewport - it should really be more like `CropToViewport` or ViewportScissor becasue it's doing two things. In GL at least you should NOT get drawing outside the viewport when the scissor test is working.