Q. Can I eat popcorn in a theater if all the ingredients are kosher? Is it not proper for one to eat in a public place?

A. Talmud (Kiddushin 40b) teaches that “One who eats in a marketplace is like a dog. And some say he is therefore disqualified to be a witness in court.

Rav Idi Bar Avin said, the Halacha follows the opinion of the ‘Some say’ The Rambam (Hilchos Eidus 11: 5) and Shulchan Aruch (CM 34:18) both rule in accordance with Rav Idi Bar Avin’s opinion.
Although, Tosafos (Kiddushin 40b) quoting Rabbenu Tam, mentions that it only applies to bread and not to fruits. (like popcorn).

Yet, Kessef Mishneh (H. Edus 11: 5) rules that a Talmid Chochom should be stringent even when eating fruits or similar in a public area.

Rabbi A. Bartfeld as revised by, Horav Dovid Pam, Horav Aharon Miller, Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit’a