Great question — answering as the UphillTech team:
Is there a provision in the ESTA rules for my situation, or should I go out of my way to get to Canada without transiting at a US airport? Yes. The regulations state that the immigration officer "may" admit you for the balance of your initial 90-day admission after a departure to Canada (or Mexico or an "adjacent island"). The officer therefore has discretion to admit you for a new 90-day period. If your initial stay was purely for transit and you've been in Canada for two months, there's very little reason for the officer to refuse a new 90-day period. It is probably a good idea to make clear to the officer that you're planning to spend two months (or however long) in the US, to ensure...
Full answer by phoog: https://travel.stackexchange.com/a/193411 (CC BY-SA 4.0).
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