> As suggested in bug#41198, here is a patch that adds
> two new options (disabled by default):

Here is a better patch.  It makes possible to enable
outline-minor-mode in any buffer, e.g. in prog mode,
when outline-minor-mode-cycle is set to non-nil.
Then typing S-TAB will collapse all headings, and
TAB will expand the current function body, like this: