Gonna disagree a bit on the bold part. It COULD mean "...interesting discussion. I'll stick around and maybe I'll learn something.". The person leaving the gif could be just posting something to make certain the thread shows up in their "Replies" list when they check the site for unread replies. If you don't have anything worth adding but want to subscribe to the thread to learn something, that's an easy way to do it.
Other boards have a "subscribe" button where you can subscribe to a thread without posting in it. If LGM had one, I'd use it a lot. Ditto for an "unsubscribe" button where you can stop getting updates from threads that no longer interest you. Lack of an "unsubscribe button" is THE ONLY REASON I don't post in the "Current Status" thread. I wouldn't be able to stop the updates. So without a "subscribe button", I display the list of "Unread Threads" and see if any of the threads I have an interest in but nothing to say (like this one, until now 
) are somewhere in the list. But if the thread appears to be really interesting and I have nothing to add at the moment, I might post a one word post "interesting" or "hmmmm" or "+1", or crack a silly joke just to get subscribed.
I'm mostly referring to what happens/has happened on LGM. Here, typically when people do the popcorn thing (or the now outdated grill), it means "I expect this to be dramatic and I'm gonna sit the discussion out, but I want a front row seat for the entertainment."
Now it's certainly possible that you could substitute "dramatic" for "interesting" but that doesn't happen HERE frequently. Usually the popcorn gif is a sign that someone just hit someone else below the belt or someone is stirring up a controversy, knowingly or unknowingly, and it's about to go downnnnnnnnnn.... usually...
