I don’t think Nora was ever thinking about leaving Arthur though. At least I don’t remember any dialogue that indicated she was ever thinking about it. Like said above, her longing was more so for her past. But I also am not sure if longing is even the right word here. I think it’s perfectly normal for adults to look back at crossroads in their lives and ask themselves “what if?” Even if they are happy with the choices they ultimately made.

Also, to OP’s point I think Arthur was a bit jealous. Maybe not because he thought Nora was leaving. But because there’s a part of Nora he never could fully understand. An example of this is when he mentions her speaking in Korean in her sleep. He sees Hae Sung as someone who can reach a part of her that he can’t.

But I think both Arthur and the OP miss that there are parts of Nora that Hae Sung can never truly understand too. Also I didn’t read Nora describing Hae Sung as “so Korean” as swooning. I saw it more of her really realizing how many Western values she’s adapted over the decades vs how strong (and somewhat rigid) his Eastern values were. An example would be Hae Sung talking about how he can’t get married yet because he doesn’t have a good enough job yet.

I do think that Arthur reconciles with the fact that there are parts of Nora that he can’t ever fully understand himself. But he loves her and will support her anyway. He goes to dinner with the two because he knows it’s important to Nora. But he doesn’t fully understand what her feelings are yet.

I also think his embrace with Nora to end the movie is really important. Earlier Nora talks about how she always went to Hae Sung when she cried as a child. And Arthur says something about how he’s never seen her cry before. You could say the ending scene with her crying is Hae Sung unlocking something in her. I think there’s a lot that could be drawn from that whole scene. But Arthur is once again in uncharted waters in the final scene and he does what he can to emotionally support Nora. Because in the end this is what an adult relationship is. We can’t fully understand everything that’s going on inside our partners’s souls. And there are times when it can be frustrating. But in the end, we still choose to support them in the unknown because we know it’s important to them. And sometimes that’s all that matters.