Helium is a headless browser, which stays on top of other apps. This is a video player which stays on top of other apps (like Helium), and automatically takes over video playback from the browser when the video isn't visible on the screen. Z-index also provides features like see-through and click-through to make it easier to control the apps behind the player.

https://sc3w.nyc3.digitaloceanspaces.com/macrumorers-z-demo.png