macOS Sierra introduced Picture-in-picture. It’s a feature in which you can watch videos while doing another task. For example, watch a YouTube video while typing up a report. The video is displayed in a small window that you can drag around the screen. This works for most videos on the web, but not all. Here’s how to force PiP for all HTML5 videos.
If you’re watching a video in picture-in-picture mode in macOS Sierra, it snaps to the nearest corner. Even when you drag it out of that corner, it will again snap to the corner nearest to where you let it go. You can use command-drag to put the video where you want it. Bryan Chaffin shows you how.