OK, I found the relevant row (connecting from US to a foreign country) but there are a few issues:

• With that row selected you are asked for departing, not arriving airline - and amazingly Air Canada isn't an option, only their Rouge brand

• The link to Customs isn't active in Safari. When you find the Customs tab another way, it's unclear what documentation is necessary and whether you'd be "entering Canada." Annoyingly, after 8 pm you need to go to Canada Arrivals. If there's a line of people genuinely waiting to enter Canada, it's unclear whether you'd have to stand in it as a transit passenger.

Perhaps someone can speak to the real-world experience, especially after 8 pm.