NFR: level collapse/expand in left menu

I really love the learn2 theme/skeleton!

While adding pages, the left menu gets really crowded
Selecting a top-level menu-item in the left menu causes all other menu-items to [collapse](https://www.dropbox.com/s/szwc5thawgqub19/collapse-menu.gif?dl=0) (great!)
Is it possible to have this behavior for all subsequent levels in the left menu?
It would help if it would only show parents and children of the first level by default....

No file or test is named; start by locating the left-menu implementation and its existing top-level collapse behavior. Reproduce the menu with nested levels and implement the requested default visibility so only the relevant parents and children remain shown, then verify the behavior in the theme.

Written by the indexing model from the issue text.