@dojinshi To be fair, her feelings for the MC were never really established beyond attachment up to this point. He was her first real friend in a series (ostensibly) about making friends. So while I'm sure the title of the chapter is meant to imply some degree of romantic jealousy, there's also the fact that, in certain people, social awkwardness
can manifest a
very similar kind of jealousy. The idea that someone could come to take away "their" person is enough to spark the same kind of jealousy as someone stealing your crush. So you're right, it was never hinted at up to this point, but arguably, it's a logical response.
At the moment, my personal feeling is that she's jealous more so over the idea that, if the MC gets a girlfriend, he'll be too busy to hang out with her/be her friend. Of course, given how these series usually go, and the title of the series, for that matter...I'm like 85% sure I'm wrong on that particular part.
EDIT: typos.