bug#17453: Isearch doesn't work properly with Follow Mode.

[Artur Malabarba]

> I don't actually understand that patch, I'll need to study the wierd/new
> constructs used in it, like `when-let'.

Here's a more thoroughly explained version of this function, that
doesn't use when-let.

seq-find is equivalent to cl-find-if, it returns the first element
that matches the provided predicate.

(defun follow--search-function ()
  (lambda (&rest args)
    (let ((search-function (isearch-search-fun-default))
          (matched (apply search-function args)))
      ;; If this is a proper user-triggered search (and not just a
      ;; lazy-highlight search), and if the search matched, and if the
      ;; match is not visible on this window...
      (when (and matched
                 (not isearch-lazy-highlight-ongoing-search)
                 (not (and (pos-visible-in-window-p (match-beginning 0))
                           (pos-visible-in-window-p (match-end 0)))))
        ;; ... see if the match is visible on another window.
        (let ((win (seq-find (lambda (w)
                               (and (pos-visible-in-window-p
(match-beginning 0) w)
                                    (pos-visible-in-window-p (match-end 0) w)))
                             (follow-all-followers))))
          ;; If so, select it.
          (when win
            (select-window win))))
      matched)))

I also changed it to use (follow-all-followers).

> However, I'm certain that that patch will not fix all the problems
> discussed in this post.

The patch as provided doesn't fix the “highlighting matches on all
windows” issue. But that's trivial to solve by removing that
`(overlay-put ov 'window (selected-window))' line. (which I have half
a mind to do right now because I just think it's a generally useful
improvement.)

>  One way or another, isearch MUST work with the
> window boundaries of the entire Follow Mode group.

Maybe I missed part of the issue. I thought you wanted Isearch to
switch to another window if that window contains the next match
(instead of scrolling the current window). For that, you only need
pos-visible-in-window-p, you don't need to mess with boundaries.