bug#17453: Framework extending window functions for Follow Mode (etc.).

[Juri Linkov <juri@linkov.net>]

>> I see no problem for follow-mode to add follow-post-command-hook
>> to both hooks.
>
> Because this ties Follow Mode to implementation details of isearch.el,
> replace.el, and ispell.el.  It is plain ugly for Follow Mode to have to
> add-hook an obscure hook for every package that attempts lazy
> highlighting.

A hook is needed not only for lazy highlighting (in its current state),
but also for every package that doesn't use the standard command loop.

>> Adding a new hook is just a one-liner, but we have to find the right place
>> in perform-replace to call it.  I think replace-update-pre-hook should be
>> called before (read-event), and replace-update-post-hook after (read-event).
>> I'm not yet sure which one is needed for follow-mode to sync windows?
>
> At the moment there is only replace-update-post-hook.  It is called after
> point has been moved, but before i-l-highlight-new-loop is called.  I had
> to move it slightly from where your patch had put it.

Yes, you've moved it to a better place.

> Sorry, I made a typo there.  I really meant replace-update-post-hook.
> Can we somehow keep this "internal use only", so that we are not bound
> somehow to keep supporting it should the `query-replace' command loop be
> reformulated (as believe it should, ASAP)?  The same applies to
> ispell-update-post-hook, which I've been forced to introduce into
> ispell.el for the same reason.

isearch-update-post-hook, replace-update-post-hook, ispell-update-post-hook
are not just a hack, they will stay as useful hooks even after we'll
rewrite query-replace/ispell to use the standard command loop.  These hooks
are for convenience, for the users to be able to set in ~/.emacs, e.g.:

  (add-hook 'isearch-update-post-hook 'recenter)
  (add-hook 'replace-update-post-hook 'recenter)

How would you do the same without these hooks, using only post-command-hook?

> #########################################################################
>
> Anyhow, here's a status update with where I am on making isearch.el and
> follow.el work together:
>
> (i) Yesterday I rebased the scratch/follow branch on the emacs-25 branch.
> (ii) I haven't yet replaced the GROUP parameter in the windows primitives
>   with (e.g.) `window-group-start'.
> (iii) isearch.el now appears to work properly.  For this, I had to swap
>   the order of invocation of isearch-update-post-hook and i-l-h-new-loop,
>   like you said.  I restored i-l-h-new-loop pretty much to the way it was
>   prior to my experimentations.
> (iv) replace.el now appears to work properly.
> (v) ispell.el is more troublesome.  See bug #22097.  I have a problem
>   with ispell putting its *Choices* window at the top of the left hand
>   Follow Mode window.  Because of FM's sorting algorithm, this causes the
>   two windows to be logically swapped, leading to confusing results.  I
>   think the neatest solution would be to put *Choices* at the top of the
>   rightmost window, preventing this.

I believe bug#22097 is easy to fix for lazy highlighting, but what you
described above is more troublesome, and might require adding support for
window groups to ispell.el (like you're adding it to isearch.el).