You will need to install a user script manager extension to install this script.
I found a much nicer browser extension that does what this fork was meant to do. See thread: https://greasyfork.org/en/forum/discussion/38442/x
So consider this a closure of this script until further notice. If the extension goes down, then I'll make a point to actually work on this.
This is a fork of Feifei Hang's Better YouTube, based at v5.1.
You can find the repository here. Contributions are welcome.
Scrolling down on the page will cause the player to be shrunk and moved to the bottom right corner of the screen, while scrolling back up will put the player back in its original position. It's as simple as that.
Through forking this, the bitter irony of the name "greasyfork" can't be left unmentioned, as there is no built-in means to fork a script.
The original has no license, especially as of v5.1, thus it is in the public domain. As such, to respect that, this fork, too, shall remain public domain.