This is the platform demo with parts I added under the group "Scroll to."
The Scrollto sprite (InvisibleSprite) is not invisible for this example, so you can see where it is.
In your game, set it's "Initial visibility" property to invisible.
Oh, almost forgot, there is a scrollto behavior on the player too, currently disabled.
You could enable it, and the player would be able to scroll backward a half screen.
You would have to change the 2 half screen width limits in event 12 to full screen widths of 640.