oh GUARANTEED. definitely not immediately or anything, even though he projects that stoic boy confidence at all times, changbin's a little insecure about the trajectory of their relationship + what jisung actually wants for awhile so he gets a little squirrelly when people flirt with jisung. and he knows felix isn't a threat, but felix is especially good at flirting with jisung so changbin's a little possessive for a bit, but i'm vividly imagining them celebrating after 3racha drop their first mixtape like a year or so into binsung's relationship, and felix is once again shooting his shot and changbin asks them to make out and it's like this whole thing and jisung is soooo embarrassed and they get a hotel room and it's the best night of felix's life (he LOVES couples)