Thanks a lot for the PR! I have one major issue, but a couple of associated nerd snipes.

My main issue is the changing of the hover-over styling:

1. How would we feel about this being a concertina style thing where we have subsections of the menu expanding rather than popping out to the right?
2. Is there any chance you can make this code handle the case where the menu goes off the bottom of the display? (i.e. by collapsing the main list?)

So this second iteration now uses concertina collapse rather than a dropdown, I think it is more suitable. It is still recursive though so you can have arbitrary depth on the nav sections.

Importantly, hover over styling is fixed, I thought it was just lazy css originally but turns out its a pain cause we are having to replace loads of built in bootstrap stuff cause it doesnt support either nested dropdowns (as my first iteration did) or concertinas in dropdowns (as my second one does).

as for going off the screeen errr.... one sec

for quibble 2:

I have made it scrollable, which I think is important for accessibility (even though the sidebar is already scrollable).

I don't think it needs anything fancier than that.

The CSS seems to be busted on all the preview builds?

as an aside, I fixed the position the dropdown appears in the first place:

cause I cant imagine it was intended that it covered the top bar.

Thank you @alasdairwilson