Greasy Fork is available in English.
Improves YouTube's theater mode with a Twitch.tv-like design, enhancing video and chat layouts, while maintaining performance and compatibility. Also fixes the broken fullscreen UI from the recent YouTube update.
< Feedback on Better Theater Mode for YouTube
Very interesting suggestion, I will look into this idea. In the mean time, it seems that the script you linked can be used in tandem with this script to achieve the effect you want, but it seems it does break when the chat is showing and scrolling is broken in fullscreen.
Alright added the stuff you wanted. Not super sure how resilient the new features will be against YouTube's constant unnecessary updates. But do tell me if anything is broken.
Would it be possible to move the header below the video, similar to https://github.com/Zren/ResizeYoutubePlayerToWindowSize?