Bike Outliner: Outline Writing tool, lists, notes app for Mac:

I think macOS needs more “bicycles for the mind”.

Bike is small enough to fully understand. Once understood it’s flexible enough use for many purposes.

Bike is small, but designed for real work. It’s fast. It can handle big outlines that break other outliners. Bike’s also fast at the basics–opening files, scrolling views, and resizing windows. Bike won’t slow your Mac down.

Bike makes your writing open and accessible. Outlines are stored in text files using standard file formats. Bike is also scriptable. Automate and extend Bike with scripts.