When you realize these two things.
Two things:
That scene in the finale which is bellissimo. PICTURE PERFECT. Because she is about to touch his hair/head and he bends down juuust enough because he craves her touch her affection her love really that much and also so that it's easier for her to reach him. Because even fitting into each other like puzzle pieces is not enough in that moment.
But then you also realize that she is reaching out to part his hair/bangs ever so slightly, almost absent-mindedly, because she is confirming he doesn't have a scar there, like he used to in the ALT 2023 timeline. He doesn't have it anymore. As if that's the last bit of evidence she needed for her own peace of mind, a silent verification, to make sure her Seon Jae is finally, really, safe. A proof that her prayer has been answered in the end.
Of course she knows. She's been there all along, after all. But after everything she--and they both--have been through, I don't think either of them can quite believe it in this moment. That this is it, it's over: she doesn't have to look for any impending doom coming his way, and he doesn't have to fear losing her to the most heartless adversary that is time.
I cannot gush enough on the writing of this show.