I LOVE popcorn!! And I still eat it with no problems, I don't even notice a difference when I'm flaring.
I actually asked my GI about different foods and he said that there was such a wide variation among people with UC about many different kinds of foods that he tells each person to do what's best for them. So basically if it bothers you, don't eat it...if it doesn't, great, have at it. So that's kinda what I go by.