I just watched this video https://www.youtube.com/watch?v=bwNyBfCk_cw
and there's something that made me wonder. around second 18, he hovers over the portal, and gets a tooltip displayed.
That must mean that there's a 'physical' object to receive mouseover events.
That unconvinced me, that triggers would solely be handled by the server; also because there are so many other things, the client is trusted with, i still think it's possible or maybe just for some portals to be handled clientside too (collision + reporting to server -> answer from server + action initiation)