Looks like they'll try and make it to EWR tonight, but if cancelled or a hypothetical, I'm wondering in a case like this if they need to go through customs at YUL?

I've had to leave pre-clearance a few times at YUL and had to go through Canadian customs, but it might have been for logistical reasons ie no way out of transborder without going through customs. They could pull up to a domestic gate.

On the other hand, perhaps transit passengers from other countries were routed atYVR through US customs and didn't have the proper documentation to leave the transit area into Canada.

Any experiences from fellow FT-ers?