I assume you are talking about popcorn that is already popped? A silly question maybe but I try not to assume. If it is not popped, it is no different than regular corn, which they grind up in their gizzard.
My only real concern would be with the salt if you have a lot of salt on it. Unsalted would not be a problem. Chickens like many birds not associated with the seas can have problems with excessive salt.
You have two concerns with the size of food they eat, impacted crop or impacted gizzard. Both of these can be caused by long strands of food like long pieces of grass becoming balled up so it cannot leave those organs. Remember there is a huge difference in what can happen and what will happen each and every time. Don't panic over nothing. Some people on this forum are good at doing just that.
When chickens graze on living plants, they bite off small pieces. They will occasionally get a long strand but most are smaller in size. This is practically never a problem with grazing chickens. It's when they eat lots of long strands from cut grass or plants that it is more likely to be a problem and even then, it generally is not. Lack of grit in the gizzard can contribute to an impacted gizzard. That's why they need grit when they are eating certain foods.
But back to popcorn. It is not in long strands. It will not get balled up and cause a blockage. They can handle it. I believe in moderation so I suggest you don't give them huge tubs of it at a time, but that is more of feeding them a more balanced diet than worries about a blockage.
I'll mention that I've found plum pits in chickens’ gizzards when I process them for the freezer. They eat things that size and can handle it.
Chickens are creatures of habit and extremely cautious about anything new. If they are used to getting food chopped up into tiny pieces, they don't know how to handle other things. Maybe this story will help explain it.
A few years ago I gathered a pretty good sized cup of corn ear worms when preparing the corn from my garden for canning. I dumped those caterpillars near a bunch of free ranging 10 week old chicks. The bravest of those chicks would slowly approach those worms, carefully and cautiously creeping closer and closer. Then suddenly, one of those worms would wiggle. Run away! Run away! Quick! Quick!!!!
Then they would very carefully start to approach again. A wiggle! Run away! Run away!
Finally one got close enough and grabbed a caterpillar. Within just a few seconds that pile of caterpillars was gone.