Allow true maximize for YouTube app.
When I maximize, I can see both my Android menu bar (clock, battery etc) and the floating apps menu bar. It's a lot of screen real estate. Why not hide the menu bar by default after 2 seconds of playback (or some such) and show it again, overlapping the video if necessary) when the window is next touched...? Perhaps this feature would be useful for other floating apps too, but I think especially YouTube.
There are options to make windows as large as possible.
However, if you need fullscreen app, use normal YouTube and not it’s floating version.
Floating Apps is all about not to run in the fullscreen mode but use windows instead.