UPDATES Auto-Toggle has now been implemented.
-There is now a OPTIONS page with 3 new options. To go there, either click the icon in your browser, or head to Edge://extensions and click on Details for Youtube Windowed FullScreen and click the "Extension Options". Please note some Shortcut Keys may not work correctly as YouTube, or your system may block them. Most keys should work if they are not reserved.

NOTE: PLEASE RESTART Edge OR REFRESH YOUTUBE IF IT DOESNT WORK AFTER INSTALLING.
A simple and lightweight extension that adds an icon which allows the YouTube video player to fill the entire page, as a full browser player. This is especially useful for dual monitor users that may wish to continue working on other tasks with a large video playing.

In other words make your youtube fullscreen in your window
FYI the shortcut key to toggle this without clicking is "`" on your keyboard

Updates: 
1. Now works with the new youtube layout 
2. Fixed compatibility issues with MagicActions and Adblock
3. Added shortcut key "`"
4. Added Options page at Edge://extensions, for 3 new settings (Auto-Toggle, Hide Full Screen and ShortCut Key)
5. Should now load more consistently.