Beyond the browser's little box
Picture-in-picture for YouTube on desktop
The browser's built-in PiP works — until you want more than one video, or a size it doesn't offer, or the comments visible too. InfinitePIP does PiP at the desktop level, so a YouTube video is just another window to capture.
Float the video region of a tab while you work. Float two streams side by side. Keep a full channel page in a translucent panel that fades until you hover. Because it's real screen capture rather than a browser feature, it works identically for YouTube, Twitch, a media player, or anything else with pixels.
One app, native Windows, free — pay what you want.