bug#30204: 27.0.50; Changing major mode resets non-default invisibility spec

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: p.stephani2@gmail.com,  30204@debbugs.gnu.org
> Date: Sun, 05 Sep 2021 11:43:26 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > I think this is a corner use case, and if there's no easy way of doing
> > something better with it, we shouldn't do anything.
> 
> Perhaps we should have a mode that even more fundamental than
> fundamental-mode, that does a real cleanup of the buffer (removes all
> text properties, overlays, etc), and just document that there are some
> artefacts that may remain otherwise when changing modes?
> 
> We could call it...  fundamentalist-mode.

We could.  But my understanding of the original report is that it
isn't limited to Fundamental mode, it expects a switch to _any_ mode
to do the same.