Greasy Fork

I'm planning to create my own custom crawler, and I would like something like this (see image). A screen-following box on the top-left where you can place just about anything on that as if that is an HTML overlaid on the twitter UI. In the most simplest way possible, just a checkbox, that remembers its state when leaving and returning to twitter.

It displays similarly to the Wayback Machine's toolbar when viewing archived pages. It's on top of everything, and follows the screen when scrolling.

Any stuff on there using JS such as a checkbox using onchange will use the JS from the GM script.

https://github.com/colejd/guify

https://github.com/dataarts/dat.gui

Umm, these don't look like they spawn a lightbox on top of the page with fixed scrolling. I only know mid-level of javascript, not the advanced stuff. I need an example of doing what's shown on the image.