Apple added a new aspect to Auto Layout called the Safe Area in iOS 11. Based on the Safe Area, developers should confine content and critical UI elements to parts of the screen where they wouldn't be blocked or interfered with by the hardware or system software. For other iPhones, the Safe Area was essentially the entire viewport. On the iPhone X, though, it's more complicated. In portrait mode, the Safe Area blocks off part of the top of the viewpoint and part of the bottom. The top is to accommodate the status bar and the sensor housing, while the bottom gives a wide berth to the home indicator.
The home indicator is a thin bar that is almost always present at the bottom of the screen to indicate to the user that they can swipe up from the bottom to exit the app or reach the multitasking interface. Since this functionality was previously performed by the home button that sat below the screen, you could see the indicator as the new home button in a way. Apple allows developers to enable an auto-hide behavior for the home indicator, but only for viewing full-screen passive content like videos.
Alternatively, developers can activate Edge Protection. This gives the indicator a more subtle appearance, making the user perform two swipes instead of one to get out of the app. This is recommended if a swipe from the bottom is a core part of an app’s functionality, though obviously the best solution is to change that interaction to something else when possible. Apple nevertheless recommends vertically scrollable views extending all the way to the bottom of the display despite the indicator’s presence.
Developers who put UI elements like navigation buttons at the very bottom of the screen might have to move them if they previously rested outside of what is now the Safe Area. Galley Foods' Anderson said that despite his relatively easy transition, this was the biggest problem he encountered in his app: