bug#81632: 31.0.91; outline-view-change-hook is marked obsolete, without an adequate replacement

On Tue, Aug 18, 2026 at 4:16 PM Stefan Monnier <[email protected]>
wrote:

> > The hook `outline-view-change-hook' was deprecated in 29.1 in commit
> > 53b1e6f96cb03a461061e1e21cd185dd8a82ce6b. However there does not seem to
> > exist an adequate replacement. Sometimes there is a need to run some
> > code when folding or unfolding sections of a buffer.
> >
> > For instance the new `markdown-ts-mode' has this comment and code:
> >
> >     ;; NOTE: `outline-view-change-hook' was obsoleted in 29.1 (commit
> >     ;; 53b1e6f96cb) on the grounds that only lazy-lock used it, but no
> >     ;; replacement was provided and outline.el itself still runs the hook
> >     ;; in 10+ places.
> >     (with-suppressed-warnings ((obsolete outline-view-change-hook))
> >       (add-hook 'outline-view-change-hook
> >                 #'markdown-ts--outline-view-change nil t))
>
> I wonder why this is needed.  I see the following comment:
>
>                ;; Don't create image overlays for nodes inside
>                ;; folded (outline-invisible) headings, since the
>                ;; images wouldn't be visible and could interfere
>                ;; with the folded display.
>                (not (markdown-ts--outline-invisible-p node-start)))
>
> The "images wouldn't be visible" bit seems unimportant (jit-lock usually
> skips invisible text, so it shouldn't be an issue resource-wise).
> Is the problem that the images are displayed even in text that's marked
> as invisible?  That would itself sound like a bug, tho I can imagine
> it's debatable and not easy to fix in the redisplay code.
>
> > Would it make sense to revert the deprecation or offer an alternative
> > API to monitor folding and unfolding? Thanks!
>
> I see it's used in a few other packages as well, but it's almost always
> ill-adapted to the needs because it doesn't say what's changed, so the
> clients end up refreshing the whole buffer every time even when we're
> just (un)folding a particular node.
>
> So, I'm in favor of introducing an alternative API
> `outline-view-change-functions` which receives some useful info such as
> a BEG..END range.
>

+ Rahul who worked on outline markdown-ts-mode integration.