It's unfortunate that you didn't post a full size, full resolution image. We can't make out any fine detail in your reduced resolution image. It would also have been good to know if the band is invariant in size as you make screen adjustments or display primary colours on screen. Or if it shows up in downloaded photos?
We are of course seeking a pattern or otherwise trying to narrow down the potential causes. Since we don't have much to go on yet, we can only guess, it looks to be a hardware defect. But there isn't enough information to be certain.
It would still be useful to look for a pattern in whether the band varies at all with different colours and brightnesses on screen. You can also use the dialer code *#08# and display red only, blue only, white only, black only, etc screens to get a better sense of which pixels are apparently contributing to the band.
Also try backing up the phone (to save your user files or installed apps), then revert to 100% stock firmware temporarily, followed by a factory data reset. If the problem persists it is all but certain to be a hardware issue. Claim it under warranty then if you can.