IPhone 6s what is this? A glitch or a feature?

From time to time I'll be browsing on Safari or on my messaging app and this happens. Where the app I'm on just moves down and there's a black space covering up the upper part of the phone. Is this some sort of feature or glitch? If it's a feature, what's it for?
This is easily fixable by pressing the home button. IPhone 6s what is this A glitch or a feature - 1

That's a new feature. It's supposed to move the top half of the screen closer to your thumb so you can press buttons with one hand.

It's triggered by touching, not pressing, the home button twice quickly.

That's Reachability. When you double tap (not click) the Home button it brings down the displayed content to half to be able able to reach the top easier which was compromised for most because of the bigger displays for the iPhone 6, 6 Plus, 6s, 6s Plus.

Feature. Not glitch.

New feature called Reachability.

  • My iPhone 4S currently encountered a software glitch? The Wi-Fi will not turn on. I never turned it off, it just randomly shut off. Whenever I try to turn it on, it stays grey and won't switch to on. My iPhone is running on software 6.1.3. I have read elsewhere that this version of iOS is glitchy. I have tried everything to fix it, but nothing seems to work. If I take my phone to the Apple store, don't they HAVE to replace it because it's a software glitch?
  • ITunes glitch trying to download ios 8 for iphone 5s? Once i click update on iTunes, the software update timer in the top of iTunes just keeps increasing by 2 hours for every second that passes. So after about a minute it says 120 hours remaining. It just keeps increasing until I click manually stop the update. How do I fix this?
  • Is it normal for my iPhone 5 to glitch occasionally? - 1 It glitches now and then. Not bad, but I thought this was normal. My draw free app shuts off every time I try to use it, I reinstalled it. But, anyway, that's not the only glitch. Just wondering if that was normal and what caused it.
  • Iphone won't turn on (power efective glitch)? I was kinda retarded and sendt the Power efective troll in a group chat on fb, and my friends phone turned off and won't turn on again!