Others are complaining about similar incidents with their Galaxy S7/Galaxy S7 edge, with the false key press taking place usually not too long after the phone has been unlocked. A Galaxy S7 owner with this issue called Samsung, and the manufacturer said that it has sent out a 60MB update that might fix the problem.
Samsung also said that if the update doesn't help, a good ol' factory reset might do the trick. That seemed to end the issue for one of the affected owners. After performing a factory reset on his handset, he has unlocked his phone more than 40 times without the Multi-Window pop up appearing on his screen.
If you don't mind running a factory reset, that seems like the cure for this issue. Just make sure you have everything backed up before you start.