From previous discussions on this topic, there are many different opinions on how to address this situation. Personally, I'm not going to get too upset if the passenger is slightly 'over my line' due to their size as long as they are pleasant. What really bugs me are chaps who feel that it is their 'right' to take all of the armrest for the entire flight--even in F. I usually don't use both armrests but will when I feel the other passenger is really taking advantage of my space.

When one guy was getting settled, he place his laptop on the shared armrest in F to the point that it extended way over my lap (I couldn't cross my legs without knocking it). I was amazed when he didn't move it after a few minutes and opened his book instead. Having already accidentally hit it with my knee, I finally asked him to move it. He was pissy and acted like he didn't want to relocate it to his side of the seat; however, he decided it was to his advantage to move quicker when the FA brought my drink and I sat it on top of his laptop. I had no where else to put it--no little "extender trays" on this flight and said laptop prevented use of tray table.

Since similar situations are somewhat common, I'm now more proactive. I usually place part of my elbow on any armrest until all parties are settled and I can see what kind of attitudes I am traveling beside that flight.