iOS: Why 50% of the screen becomes black ?

Over the past few months I occasionally got a weird bug that the content on my iPhone screen would be pushed down and half of the screen becomes black (like in the screenshot).

For few weeks while using iOS beta version I was sure it was a bug but when I updated to a public release version it still happened.

Before I had the time to do some research Maret found out the logic — there’s a feature called Reachability which provides easy access to the top half of the screen.

To turn off or on this feature, simply followup these steps.

The current UX flow feels more like a bug than a feature. Am I the only one who thinks so?