@trh
I still love (and often think about) this document navigation paradigm from Paper.
It's an excellent example of productizing byproducts.
Not only is it automatic based on my markdown activity on the page, it *extends the functionality of work I've already done*. When I apply hierarchy to the document, Paper makes that hierarchy navigable elsewhere.
"But no one asked for that! What if someone doesn't want that 'clutter' in their view?"
Great question. It's just visible enough to know it's there and readily available with a hover interaction, but it's not so obvious that it's distracting or diruptive.