>>94203470>Assuming it's even going to happen at all
lol>will Marco break up with Jackie or will Jackie be the one to end things?
Marco wouldn't break it off with Jackie until he develops feelings for Star, which would be a long way off, realizes that he's being unfair to Jackie by ignoring her so much, which isn't likely to happen, or if Jackie makes unreasonable demands of him to continue their relationship, like telling him he can't be friends with Star anymore. Which doesn't really fit Jackie's character either unless something really extreme happens, like Star kissing Marco, which Star almost definitely wouldn't do unless they were in some extreme situation, like if she thought they were going to die.
Jackie currently has good reason to feel that her relationship with Marco is threatened, even moreso if Marco didn't tell her he was going to Mewni and ended up just ditching her on Earth for at least a few days.
Still, she would want to trust Marco, and their relationship had been steady if pretty uneventful for a few months up to this point. She's not going to break up with him right away over that, but I expect her to confront Marco about how he keeps ignoring her in favor of Star, maybe even ask him how he feels about Star which is a whole other can of worms.
The way I expect things to play out is that Jackie keeps Marco anchored on Earth, they start regularly spending time together (not just whenever Star pushed Marco into dating his girlfriend). Marco still visits Star on Mewni at least a few times a week, but it kills her a little inside every time he leaves. I expect at least one awkward encounter where Star visits Marco's house and runs into Jackie.
Eventually the plot creeps up again, probably with Eclipsa being released, and Marco won't be able to just be on Mewni part-time if he wants to help Star.
Jackie draws the line at the idea of Marco leaving AGAIN, tells him she can't deal with this kind of relationship, and breaks it off with him there.