Regarding LABs app FC, FWIW:
On a Moto X4, Oreo, LOS 15, Magisk device the FC occurs very shortly after the UI loads. During the brief period before the FC tabs can be switched and some scrolling can be done. It seems as if scrolling causes it to happen sooner but I'm not certain of that and even if left untouched the FC occurs within seconds.
On. Moto G4, Nougat, LOS 14, Magisk device, FC does not occur nearly as quickly, in fact in most tests not even after some minutes left untouched or changing tabs and doing a good bit of slow scrolling. One thing that seems to precipitate it is an abrupt swipe up or down scroll.
This might just be the different OS and web components reacting differently to the underlying issue.