iPhone X - Safe Area does not achieve full-screen experience?

Question

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:

Answer 1

Just change your bottom constraint First Item to SuperView