> I attached a revised version of the `group-function` for
> `completing-read` patches, implementing the suggestions by Juri Linkov.
>
> 1. The guard variable `completions-group` is not checked centrally, like
> `completions-detailed`.
> 2. The vertical completions format separates the groups completely using
> full-width group separators, instead of inserting the titles within the
> vertical flow.

Thanks!  Now separating the groups completely using full-width group separators
for the vertical format allows enabling outline-minor-mode and using
TAB/S-TAB keys to hide groups for better overview in a large buffer.

I tried (setq outline-regexp " ") to match the space character
at the beginning of group titles to find outline headers,
and the result is quite nice: