How about in the dark, on a bedside table when your head is half obscured by a pillow?
Nintendo’s “New 3DS” has facing tracking glasses free 3D - for it to work well it has to keep track of your eyes and other facial features for the effect to work from various angles. (The 3DS didn’t have the face tracking, and as such you had to hold it perfectly still in one sweet spot for the effect to work.)
The way it works at night is with a little infrared light next to the front camera - if the light level gets too low you can see the dull red color show. And it still worked just as well! Apple would likely use this type of tech.