Nested Iconified Side Navigation Menu (Full Button Demo)

Collapsible and expandable side navigation menus with icons have long been a known UI concept (example here).

What about nested collapsible and expandable side navigation menus? I first thought about this kind of navigation menu in about 2015, but was not able to find nor figure out a UI solution to this problem. Since then I've had this in the back of my head, but not actively searching for a solution. In the beginning of 2020 I developed the CSS only navigation menu framework HyperNav, during which I revisited the concept of nested collapsible and expandable side navigation menus. Finally, I was able to produce a solution that just works (not talking about the technical implementation here, but rather the UI and UX experience).

What do you think about this kind of navigation menus (see the demo on the left)? And have you ever encountered this kind of navigation menu before?

Demos:
Full Button Demo
Split Button Demo

The Full Button Demo features only "full buttons", which means that the expandable icons function expansion buttons.
The Split Button Demo features only "split buttons", which means that the expandable icons function as expansion buttons and links.

See how the behavior of "Simulated Page Content" and of the navigation menu to the left changes depending on which of the demos you are viewing. For example, the Full Button Demo will never show "Stars" as the title of the yellow box, while the Split Button Demo will.

HyperNav GitHub Repository

https://github.com/Acmion/HyperNav

Note:
The content on this page is primarily about the UI and UX considerations and not about HyperNav. This page also features some enhancing JS that is not a part of HyperNav.

Simulated Page Content