It works for what it says it's supposed to, but it's pretty clunky.

There's a dropdown menu with only two options of on or off, which means there's no need for a menu, the button itself should be a toggle. There's no option for a hotkey to make it easier. The author mixed up the names for the icons, the white icon is on white and black icon on black (if you change the file names it's correct).

Unfortunately it doesn't work with Twitch but it works with YT.
The "scroll to the top of the page" is also a problem.
But beside this it's a cool addon and I'd like to see some updates/fixes.

it does lock the scrolling of the webpage however it also first scrolls the page to the top so it is only useful if what you want on screen is on the top of the webpage.