I have researched this multiple times over the past few years (since I started using and loving/hating OneNote). I believe the short answer, at the moment, is that it is "not possible" but here's the question:
How could one replicate OneNote's collapse/expand hierarchy functionality in Google Docs? In OneNote, you can hover over a parent of a child element (indented paragraph or list item) and an icon appears to the left of the parent element. Click on the collapse/expand icon to hide/show the child hierarchy beneath the parent.
The most common answer I've found is that you can use hierarchical elements in Google Docs, along with a linked Outline -- like h1, h2, h3 -- to create a sort of left-hand navigation/table of contents to anchored headings. This is a good, useful feature but not what I want.
Could a script or Chrome extension provide this? I have not seen one developed, but does anyone know if there are immovable barriers to creating such a script or extension?