Offcanvas inside containers other than viewport

Created by: iainhallam

In some user interfaces, it's possible to make a drawer appear using any element with appropriate classes as the container, rather than the viewport. I'd like to use this to allow properties panes to easily be associated with different parts of the page, rather than needing to go to the window edge all the time.

A codepen for the sort of thing I'm after:

https://codepen.io/iainhallam/pen/vYywVev

If this is already possible, could it be added to the documentation, and if not, could this be a feature request, please?