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

[Juri Linkov]

> This means the use of `query-replace' whilst Follow Mode is enabled is
> not going to work properly, without some radical change in replace.el.
>
> Probably the smallest change would be to invoke new hooks
> `pre-replace-command-hook' and `post-replace-command-hook' from
> `query-replace''s command loop.
>
> A more satisfying change would be to get rid of `perform-replace' and
> use Emacs's command loop the way Isearch does.  This would probably not
> be all that difficult.  Do you know if there's any special reason
> `query-replace' implements its own command loop?

The patch in bug#20430 awaits the possibility of helping to fix this
problem.  It adds a new hook replace-update-post-hook that is like
its isearch counterpart hook isearch-update-post-hook is the right way
to handle display updates like syncing follow windows, etc.

Together with changing the order of calling isearch-update-post-hook
and isearch-lazy-highlight-new-loop in isearch-update, adding
follow-post-command-hook to isearch-update-post-hook, and adding
follow-post-command-hook to replace-update-post-hook to handle
follow-mode in query-replace will comprise the least radical change
just before the next release.

Do you see a shortcoming of this course of action?