Why is this even a problem? We don't need a "rule" to address this. It's a shared resource, and we first learned how to share back in kindergarten. (Apparently some people have learned better than others.) On a plane this goes for the armrest, aisles, overhead space, or even standing in line for the loo.

Etiquette simply means "politeness". In other words, not deliberately doing something that harms another person or makes them feel bad. Should you do something unintentionally, it's not impolite to apologize.

Just ask your seatmate (politely, of course) how they feel about the issue, and come up with a way to SHARE, Unless you feel that the simple act of asking will create an even bigger outburst, then "etiquette" calls for you to keep your mouth shut.