If you can’t understand, try drawing it on a piece of paper. A black bar straight across the end of the screen with the sensors will form two right angles with the adjacent straight sides of the screen/device. The end opposite the sensors will have rounded corners because the screen follows the curves of the device itself. Held in portrait, the square corners are at the top (or bottom, if the phone is held with the sensor end down). In landscape, the square corners may be at either the left or right, depending on how the user holds the phone.
The Apple application programming interface since the Lisa has allowed for either rectangular windows (Rect) or rounded rectangles (RoundRect). There is no standarkd means for handling half-rounded rectangles.
If an iPhone X app programmer defines his main window as a RoundRect, the system software provides standard means for handling all four possible orientations, although there will be a notch out of the sensor end or side of the screen. That is what Apple recommends.
If the app programmer defines his main window as a Rect, the square corners at the sensor end of the screen will eliminate the notch. Great, but it will also define the opposite end of the window as having square corners. That will not match the physical shape of the screen, meaning that there must either be a matching black bar at that end (a virtual bezel), or the extreme corners of the window will protrude offscreen and make that content invisible. That would look at least as bad as the notch.
The only other alternative is to force every individual application programmer to come up with her own nonstandard solution for fitting content into a screen with one square end and one rounded end… with four possible arrangements depending on how the device is held. Some solutions could be very ugly, and no solution would be the same as any other, making the code difficult to maintain.
What part of that explanation SUCKS?
Source : http://macdailynews.com/2017/10/28/apple-reminds-developers-to-design-around-iphone-xs-notch/366