Re: expandable headlines in text editor?
org-mode in emacs does this, sort of.
When you edit a file in org-mode, lines beginning with * A Title, ** A Subtitle, *** And So On are understood to be headlines that you can move to and from, as well as hide (with the contents) and show. org-mode also has plenty of other features that you may or may not find useful.
However, it's emacs... It's a wonderful editor but it does have a learning curve.
This is the first age that's paid much attention to the future, which is a little ironic since we may not have one.
-- Arthur C. Clarke