Mcdonalds decided it's "unsafe" to run their app in private space of Android. In literally the most locked down part :) Marketing must have gotten a nice bonus for that mental effort.
I can run banking apps like that, corporate apps like that, but I can't show a QR code to order happy meal.
I've read about a few incidents where people could order for free or below cost so I'm not surprised their app developers are a little paranoid.
You can't even use the McDonald's app if you have an overlay. I use KineStop and in the car I'm already choosing what to order and I can't click anything until I turn off KineStop...
In comparison the Burger King app works without problems and is very fast.