Simply put, knowing how advanced org-mode can get with folds, this is likely trivial to do. (If you haven't played with it, I'd recommend taking a look. I don't have perfect examples, but http://taeric.github.io/DancingLinks.org is an example. Open that in emacs with org-mode and you will see that all of the source blocks can be folded together. For me, the source blocks are correctly colored, as well.)
Quickly searching, I found https://www.emacswiki.org/emacs/FoldingMode, which seems to be a large start to this. There were also some other ideas that seem to get close.
The basic idea I had was to just make a few functions that would:
- Fold from current location to next #endregion
- List all region/endregion's in the current buffer
- Fold all region/endregion's in the current buffer
Edit: It occurs to me that I didn't show how to actually hide text, which I think is your question. Fastest way I can see is this (run this in your scratch buffer to see it in action):
(let ((o (make-overlay 5 120)))
(overlay-put o 'display "---hidden---"))
(let ((o (overlays-at 5)))
(mapcar 'delete-overlay o))
I already pondered if it wouldn't be possible to merge multiple "folding strategies" if one would build some kind of interface with which multiple routines could basically communicate folds in a buffer.
Say you have one routine scanning the syntax of the according language, one looking for folding markers and one for manual (Vim-style) folds. I guess it technically should be trivial to use all of them provided they are nested and don't intersect in weird ways (the latter could be resolved with some simple priorities).