On my recent flight, the guy next to me said the middle seat gets both armrests, then immediately spread out with his laptop, iPad, and phone. His elbows were way over into my space, so I asked him to move. He claimed he was “allowed.”
So I took the armrest instead. He called the flight attendant, who said there’s no rule about armrests and told us both not to use them.
After that, I just sat back and enjoyed the rest of the flight.