This is the bug that sort-lines since Emacs 25 has not handled collapsed
(invisible) lines properly (keeping them with their parents) when sorting.
Prior to Emacs 25, sort-lines handled outline sorting properly.

It should be a fairly straightforward thing (relative to the complexity of
other Emacs issues) since it is well understood and a potential patch has
been offered.  Eli suggested that any change be limited to outlining modes
to limit its effects.

​I cannot work on it myself right now and that is why I am asking someone
else to pick it up.​


I think this is a bug that should be eliminated before Emacs 26 goes live
​ so it does not impact people with large outlines who might sort them and
get the wrong results but not notice until their data has been corrupted
for a while and they can't get back to the original proper hierarchical
associations.​

At the very least the present behavior makes no sense for outline modes
that use invisibility for collapsing.

​Bob
​