Wide Stage Chrome extension for YouTube
YouTube keeps the player in a narrow column next to the recommendations, and fullscreen takes over the whole screen. Wide Stage is a small Chrome extension that fills the browser tab with the player instead.
Press ` on any watch page, or click the new button before the fullscreen button. The sidebar, description, and comments disappear and the video stretches to the size of the tab. It keeps its aspect ratio, so it fills the screen when the shape matches and shows bars when it doesn't.
Under the hood it's one class on the <html> element and a stylesheet that pins the player to the viewport and hides the extra columns.
It's open source on GitHub. Since it isn't on the Chrome Web Store, download the ZIP and load the extracted folder from chrome://extensions with Developer mode on.