Wow. OK, thanks. Google AI says: "Compersion is a feeling of joy, happiness, and satisfaction experienced when witnessing a partner's happiness and positive interactions with another person, often in the context of polyamory or other non-monogamous relationships. It's the opposite of jealousy..."

That's the one. I only learned it recently also so I probably should have included a link to a definition. Some definitions make it more about partner's than others, which makes it an imperfect match for schadenfreude, but I still like the symmetry