SetViewport `sets a viewport`. A viewport is NOT a constrained area within which graphics are drawn. A viewport is a matrix transformation from world coordinates to window coordinates. It has nothing to do with cropping the graphics operations. That's why you also have to use a cropping feature such as a scissor window. In BlitzMax SetViewport does both - it sets up a viewport transformation matrix and switches on the scissor window to clip graphics. You must have both to prevent drawing outside the area.
ou can't cover every possibility unless you have You just contracticted yourself again, so I still can't grasp what you're saying. I think you're saying that BlitzMax terminology is different from OpenGL terminology, which isn't surprising since OpenGL isn't the only game in town. Or indeed in BlitzMax.
And what if, like Sledge says, they choose for their driver to clip fullscreen software. Do you have a way to check for this, or will you now have the automatically clipped space along with your black rectangles?
Check for what? There's nothing to check for. If what Sledge says is true, it doesn't affect you because you're not running in a 4:3 resolution, you're running in a widescreen resolution and faking 4:3.