Set scrolling element to something other than the window

Hello everyone!
Is there a way to change what the scrolling container is from the default (window)?

The main use case I see is a website where the view is wrapped in a scrolling div (and so, the window/document isn't scrolling).
In my case, I'm doing this to have a css `perspective` on the scrolling div but still being able to have a `position: fixed;` background below.

No source files or tests are named in the issue. Start by tracing how basicScroll reads and listens to the window scroll position, then check how a scrolling div could be supplied instead. Done means parallax updates work when a div scrolls while the window remains stationary, including the reported perspective and fixed-background use case.

Written by the indexing model from the issue text.