The extension automatically makes the YouTube player fill the entire screen and is extremely lightweight and simple to install. You can now simply click the icon to enter fullscreen mode. If you have the YouTube app installed, it will automatically enter a fullscreen mode for you. If you want to exit fullscreen mode, click the icon again or press the Esc key.

This is a highly recommended extension for those that use dual monitors and may need to continue working on other tasks while watching a video.