This is an interesting idea, and I can see why you’d want it.
In a future iteration, we are intending to have a sticky menu at the top of the page that travels down with the content, but this would contain the immediate page menu (i.e. the stuff under “In this article”), not the sidebar for the whole page set.
Perhaps we could add a “Scroll to top” type link to this sticky menu, which could be shown or hidden as required in the page options (it would be more useful for this type of page, but not so useful for a reference page where the page is shorter and the structure is always the same).
I’m pinging our front end dev @Stephanie_Hobson too, to see what she thinks.