The simplest case would be someone creating a portal to a pocket dimension (or perhaps the door and dimension both). Is this under the jurisdiction of the country where the portal appears?
I suspect it would matter a great deal how defensible and how easily sealed such portals are. A proper superheroic type would let the authorities have authority if he agreed they had jurisdiction -- but there's a reason why in internationl law, for centuries, nations recognized authority over coastal waters to exactly the range that a fired cannon could reach.
More complex ones would get -- more complex. Imagine two superimposed regions, with various portals -- which, perhaps, open at selected times, or worse, to selected people.
It would get interesting.