TSA-approved backpacks can zipper all the way open in a way that isolates the laptop compartment so that the scanner can see the laptop without any further tampering with the backpack and its other contents.
Some backpacks conveniently have a tablet compartment that’s opposite the laptop compartment so that when the pack is zippered all the way open, the tablet is isolated before the scanner, too. Note that TSA-friendly devices can be opened like normal backpacks, too; the open-flat position is just a convenient added feature.