I fixed my problem, this may fix yours!! So ive been having this issue since yeatersay, all of a sudden my 3 system buttons at the bottom of the screen no longer responded. At first i would turn screen off and on and got it back for a minute but after doing it a few times it no longer works.
Just now i found that by changing the resolution i get full nornal function (and a slightly-unnoticeable smaller screen). The only down side is the top menu wont slide down just by swiping top to bottom, you have to press on the top bar then swipe down.. Not a big deal. Regardless, i thing an ap must havr changed my reaokution problem. Let me know if it fixes your problem.
I used "resolution changer pro" by nexter.
Also note I have rooted my phone a few weeks ago, this may be required in order to work.
(Rooting took a minute and i didnt have to delete or restore anything)