The new HIG for iPhone X available here, specify: "Provide a full-screen experience. Make sure backgrounds extend to the edges of the display, and that vertically scrollable layouts, like tables and collections, continue all the way to the bottom."
Now I'd like to understand how to accomplish that with Xcode 9 GM, since it seems to me that the only view allowed to extend to the whole screen is the UIViewController root view, and that whenever I try to drag constraints for a view above that, i.e. WKWebView to the root view, the constraints get actually connected to the safe area, leaving both the top and bottom areas empty as displayed in the storyboard here:
Please note the answer here specifies to use the safe area, but that doesn't work because using it results in the following simulator result where WKWebView is NOT extending to the edges of the screen:
Just change your bottom constraint First Item to SuperView
Actually I believe the answer is this: to accomplish full screen we should NOT use safe area support. After I unchecked Safe Area Relative Margins and Safe Area Layout Guide on the view in Interface Builder Size Inspector I got the expected result:
In fact I would say that the idea of Safe Area is that of an area which will for sure not be overlapped by any of the system icons, or rounded borders, full screen is the opposite of it.
来源：https://stackoverflow.com/questions/46275330/iphone-x-safe-area-does-not-achieve-full-screen-experience