If I open YouTube in the Chrome browser in Windows, there is a button for “theater mode” which seems to do what you want without needing any sort of extension.
Dewey - strange, in my YouTube, in the lower right of the viewer, there is an icon for “Cinema mode” which doesn’t change any of the scrolling characteristics. Can’t find “Theater mode”.
From what I’m seeing, cinema mode and theater mode are the same thing. Try this. Open Chrome in incognito mode and try again. Does it work now? If so the problem may be an incompatible extension. (Normally incognito mode does not launch extensions, but if the extension is still running there, this won’t help.)