Not sure the complete extend of this. If you add a clip, I used this one: https://media.html5media.info/video.mp4 With chrome, you can play and pause, but seeking doesn't seem to work. If it ends, you can not seek back to the start, so it's stuck at the end.
Popcorn Maker is no longer under active development. https://learning.mozilla.org/blog/product-update-for-appmaker-and-popcorn-maker