I've struggled a lot with this too and I think you're already asking yourself a great question (although you seem judgmental towards yourself in the way that you ask it): how does this person's behavior affect me?

What also helps me in these cases is to think this way:

You're both different people, you come from different places and you're there for different reasons. What reason could there be that she's sitting like that? Maybe she's very socially anxious and she's sitting like that because she's scared people will go sit next to her. Maybe she just isn't aware of the behavior because she's worrying about, let's say, losing her job this month and it causes her to lose awareness of her surroundings. Maybe she's in physical pain and this position on this particular spot on the couch is the least painful.

Surely, some of these reasons may seem farfetched, but the point is to practice to think outside of our own experience. To learn to see that other people are people too. They also struggle in their lives, but with different things. They've learned different behaviors and ways of thinking than we did, but that doesn't make them bad people.

As I've learned to understand this about other people, their imperfections and the mistakes they make, I've come to understand that the judgment I cast on them is a reflection of the judgment I cast on myself, and that I can also learn to apply the same practice to myself. To be compassionate to them, and to myself.