People can open your phone with Touch ID while you're sleeping.
I'm guessing that Face ID will be much more sophisticated than that. It will likely be sensitive enough to know when it's being turned at a verified face and then away, back at the person who's using it which will red flag it as being used by someone other than the authorized user. This way, someone can't just point it at the owner to unlock and run off with the phone.
Erm Nope! Again you people are wrong! As always! So don't embarrass yourself.
With Touch ID if someone tried to use my finger while I am sleeping in order to unlock my iPhone then I will feel them do so. I will then wake up and punch them into next week.
Next that would be a major problem if I unlocked my iPhone 8/x/edition and turned the phone around to show something to a friend then back again.
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That's an easy fix. Press your thumb to an onscreen button to pay with ApplePay. At that point, it'll Face ID you and will remain verified as long as your thumb is on that button. When you let go, it'll be deauthorized until you press it again and it can check to see if you're the one using it.
Another way would be to have an Apple Pay button that you press when the phone is in front of you and becomes authorized and once you let go, gradually goes back to a requiring authorization state which gives the user the idea that it's a limited time thing. So that way, if you're going through a drive through and need to stick your phone out the window, you can authorize it, then tap the phone to the terminal without you in its view.
This is a UI problem, not a technology problem.
Soooo "Press your thumb to an onscreen button to pay with ApplePay" - In other words just like I do at the moment with TouchID.