At my theater, they brought the lights up before the panel discussion. I want to say they did the same thing last year, but I'm not 100% sure.
It was late (8:30 p.m. on a weeknight??) and some people would've left anyway - my friends knew there was a panel discussion but left after the movie ... and they're both retired, just early risers with kind of a long drive home.
So I don't know how many people really left because they didn't know about the panel. They did mention, after introducing the panelists in the opening bit, that they would be back after the movie.
