Compersion, like polyamory, is brought out by different things to different people.
You definitely have the basic definition down, and the answer to what you say is, from my perspective "it can be all of the above". Because compersion is that feeling of joy when you partner is happy and they are not doing something directly with you. Often when people see this they feel envious or jealous, because they feel that it is their "right" to have their partner only happy in relationships with them. The concept of compersion is the opposite of that, and so it can encompass many many types of moods and activities.
So if that means joy at your partner having sex, that's fine, if it means joy at your partner being out on a date with his OSO that's fine too - it's all good, as they say.
Oh and one other thing - if your partner gets off being in the room while you are having sex with your OSO, then that's just fine, as long as all are comfortable with it - I don't think many on here would want to dismiss is as a "cheap thrill".
And welcome to the forum!