This may sound crazy, but i think the capacitive buttons, that were below the screen on the prototype device, are stiil there ( see the pc attached ). I think is the same thing like the HTC did with their HTC One logo "button". The HTC devs made it possible by modifying the kernel.
How i "discover" that? Simply press the home button on the screen and drag your finger gently below the edge of the screen and you'll see that the home button is still pressed.
Let me know that this is happened to you too?!
I don't have any free time to try to see what codes/lines are in the kernel that could possibly turn that buttons on.