Chosen Solution
I recently bought an iPhone 6s from someone. From the home screen, everything works fine. But once you go into an application(any application) the bottom part of the screen goes unresponsive. That’s basically it. The bottom of the screen works when you’re on the home screen perfectly, but the bottom doesn’t work at all when you’re in an app.
It’s hard to imagine what kind of software issue could cause this. If you haven’t already, start by doing a hard reset (Home & Power for 10 secs). If that doesn’t help, do a backup via iTunes and then restore the device to get a fresh version of iOS. If that doesn’t work, then consider trying a replacement, preferably known-good screen. It may still be hardware related.
