When opening a video in full screen from Youtube (and presumably elsewhere), Orion currently echoes Safari's behavior of adding a new "Space", basically creating a new fullscreen "window" just for the video. This makes watching videos on both browsers feel slower and even a little nauseating to me due to the amount of motion and how laggy the animation is.
On Chromium and Firefox, videos stay in the same window when going into full-screen. In my opinion, this is a much better way of handling full-screens.
(Not marked as a 'bug' since it's also present in Safari. I imagine it's intended by Apple, but I don't know who actually prefers it.)