It’s very simple, if someone overflows into your seat then you let the FA come up with an acceptable solution.
Having once in the past, when I was much younger, accepted the situation on a 10 hour flight and arrived at the other end with a bad back because I’ve had no room to move and being forced to sit at an angle, I won’t accept it ever again. Even if that means I have to wait 24 hours for the next flight.