If a stage has a parallax background, what's the best way to make the screen loop around so that the stage continues endlessly? Specifically, I'm trying to make a highway stage in which the player has to jump on top of one car to another until he reaches the boss fight at the end, which also takes place on moving cars on the highway.
You could make the frame be as long as the level needs to be. You could also make the frame be 2.5x the size of the window and just use a detector to "throw" the player (and any other objects) to the other side of the screen.
I think the easiest way would be to use the Active System object (or similar object) and use that to create the moving background effect by [always] moving it 5 pixels left and wrapping it to x=25 every 5 frames. Obviously you don't need to use those exact values. Other objects will do the same trick, but the Active System object goes behind backdrop objects.