Chabad has the minhag to specifically eat mezanos before davening. Other chassidim may be lenient as well; I don’t know. But traditionally Breslov has been strict — Rebbe Nachman and Reb Noson were careful not to drink even water or coffee before davening.
Some people probably daven Shacharis at home and make kiddush and then have a snack before coming to shul on Shabbos, but then of course you don’t get to daven with a minyan for Shacharis.
I think I’ve heard people say you’re allowed to eat before Shabbos davening if you would get weak without food, but I don’t know if there’s a non-chassidic posek who rules that way (for a regular healthy person.)