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

[Juri Linkov <juri@linkov.net>]

> +(defvar isearch-lazy-highlight-ongoing-search nil
> +  "Dynamically bound to t in `isearch-lazy-highlight-search'.
> +This can be used by `isearch-search-fun-function' to detect
> +whether its return value is being run for a proper user search or
> +a lazy highlight search.")
> +
>  (defun isearch-lazy-highlight-search ()
>    "Search ahead for the next or previous match, for lazy highlighting.
>  Attempt to do the search exactly the way the pending Isearch would."
>    (condition-case nil
>        (let ((case-fold-search isearch-lazy-highlight-case-fold-search)
> +            (isearch-lazy-highlight-ongoing-search t)
>  	    (isearch-regexp isearch-lazy-highlight-regexp)
>  	    (isearch-regexp-function isearch-lazy-highlight-regexp-function)
>  	    (isearch-lax-whitespace

Please note that this is unnecessary because all functions that override
isearch-search-fun-function use its ‘bounds’ arg to detect whether it's
called from isearch-search or isearch-lazy-highlight-search with the value
either nil or non-nil correspondingly.

> @@ -279,8 +279,13 @@ isearch-lazy-highlight
>    "Controls the lazy-highlighting during incremental search.
>  When non-nil, all text in the buffer matching the current search
>  string is highlighted lazily (see `lazy-highlight-initial-delay'
> -and `lazy-highlight-interval')."
> -  :type 'boolean
> +and `lazy-highlight-interval').
> +
> +When multiple windows display the current buffer, the
> +highlighting is displayed only on the selected window, unless
> +this variable is set to the symbol `all-windows'."
> +  :type '(choice boolean
> +                 (const :tag "On, and applied to all windows" all-windows))
>    :group 'lazy-highlight
>    :group 'isearch)
>  
> @@ -3162,7 +3167,8 @@ isearch-lazy-highlight-update
>  			  ;; but lower than isearch main overlay's 1001
>  			  (overlay-put ov 'priority 1000)
>  			  (overlay-put ov 'face lazy-highlight-face)
> -			  (overlay-put ov 'window (selected-window))))
> +                          (unless (eq isearch-lazy-highlight 'all-windows)
> +                            (overlay-put ov 'window (selected-window)))))

This is a good option that follow-mode could set (or users to customize).