First commit to scratch/follow. Make Isearch work with Follow Mode, etc.

doc/lispref/window.texi (Basic Windows): Add paragraph defining "Group of
Windows" and new @defun selected-window-group.
(Window Start and End): Describe new &optional parameter GROUP and
...-group-function for window-start, window-end, set-window-start, and
pos-visible-in-window-p.
(Textual Scrolling) Describe the same for recenter.
doc/lispref/positions.texi (Screen Lines): Describe the same for
move-to-window-line.

src/window.c (Fwindow_start, Fwindow_end, Fset_window_start)
(Fpos_visible_in_window_p, Frecenter, Fmove_to_window_line): To each, add ar
new optional parameter "group".  At the beginning of each, check whether the
corresponding ...-group-function is set to a function, and if so execute this
function in place of the normal processing.
(syms_of_window): Define symbols for the six new variables below.
(window-start-group-function, window-end-group-function)
(set-window-start-group-function, recenter-group-function)
(pos-visible-in-window-p-group-function, move-to-window-line-group-function):
New permanent local buffer local variables.
src/keyboard.c (Fposn_at_point): Add extra parameter in call to
Fpos_visible_in_window_p.

lisp/window.el (selected-window-group-function): New permanent local buffer
local variable.
(selected-window-group): New function.

lisp/follow.el (follow-mode): Set the ...-group-function variables at mode
enable, kill them at mode disable.  Add/remove follow-after-change to/from
after-change-functions.
(follow-start-end-invalid): New variable.
(follow-redisplay): Manipulate follow-start-end-invalid.
(follow-after-change, follow-window-start, follow-window-end)
(follow-set-window-start, follow-pos-visible-in-window-p)
(follow-move-to-window-line, follow-sit-for): New functions.

lisp/isearch.el (isearch-call-message): New macro.
(isearch-update, with-isearch-suspended, isearch-del-char)
(isearch-search-and-update, isearch-ring-adjust): Invoke above new macro.
(with-isearch-suspended): Rearrange code such that isearch-call-message is
invoked before point is moved.
(isearch-message): Add comment about where point must be at function call.
(isearch-search): Remove call to isearch-message.
(isearch-lazy-highlight-window-group): New variable.
(isearch-lazy-highlight-new-loop): Unconditionally start idle timer.  Move
the battery of tests to ...
(isearch-lazy-highlight-maybe-new-loop): New function, started by idle timer.
Note: (sit-for 0) is still called.
(isearch-lazy-highlight-update): Check membership of
isearch-lazy-highlight-window-group.  Don't set the `window' overlay
property.
(isearch-update, isearch-done, isearch-string-out-of-window)
(isearch-back-into-window, isearch-lazy-highlight-maybe-new-loop)
(isearch-lazy-highlight-search, isearch-lazy-highlight-update)
(isearch-lazy-highlight-update): Call the six amended primitives (see
src/window.c above) with the new `group' argument set to t, to cooperate
with Follow Mode.

7 files changed, 838 insertions, 373 deletions

diff --git a/doc/lispref/positions.texi b/doc/lispref/positions.texi
index 72b76ce5c8f..e0496e30848 100644
--- a/doc/lispref/positions.texi
+++ b/doc/lispref/positions.texi
@@ -551,7 +551,8 @@ current buffer, regardless of which buffer is displayed in
 any buffer, whether or not it is currently displayed in some window.
 @end defun
 
-@deffn Command move-to-window-line count
+@deffn Command move-to-window-line count group
+@vindex move-to-window-line-group-function
 This function moves point with respect to the text currently displayed
 in the selected window.  It moves point to the beginning of the screen
 line @var{count} screen lines from the top of the window.  If
@@ -570,6 +571,16 @@ In an interactive call, @var{count} is the numeric prefix argument.
 
 The value returned is the window line number point has moved to, with
 the top line in the window numbered 0.
+
+If @var{group} is non-@code{nil}, and the selected window is a part of
+a group of windows (@pxref{Basic Windows}), @code{move-to-window-line}
+will move to a position with respect to the entire group, not just the
+single window.  This condition holds when the buffer local variable
+@code{move-to-window-line-group-function} is set to a function.  In
+this case, @code{move-to-window-line} calls the function with the
+argument @var{count}, then returns its result, instead of performing
+the actions described above.  Typically, the function will call
+@code{move-to-window-line} recursively.
 @end deffn
 
 @defun compute-motion from frompos to topos width offsets window
diff --git a/doc/lispref/windows.texi b/doc/lispref/windows.texi
index 357247ef433..832ced719cd 100644
--- a/doc/lispref/windows.texi
+++ b/doc/lispref/windows.texi
@@ -133,6 +133,30 @@ This function returns the selected window (which is always a live
 window).
 @end defun
 
+  Sometimes several windows collectively and cooperatively display a
+buffer, for example, under the management of Follow Mode, where the
+windows together display a bigger portion of the buffer than one
+window could alone.  It is often useful to consider such a @dfn{group
+of windows} as a single entity.  Several functions such as
+@code{window-start} (@pxref{Window Start and End}) allow you to do
+this by supplying, as an argument, one of the windows as a stand in
+for the whole group.
+
+@defun selected-window-group
+@vindex selected-window-group-function
+When the selected window is a member of a group of windows, this
+function returns a list of the windows in the group, ordered such that
+the first window in the list is displaying the earliest part of the
+buffer, and so on.  Otherwise the function returns a list containing
+just the selected window.
+
+The selected window is considered part of a group when the buffer
+local variable @code{selected-window-group-function} is set to a
+function.  In this case, @code{selected-window-group} calls it with no
+arguments and returns its result (which should be the list of windows
+in the group).
+@end defun
+
 @node Windows and Frames
 @section Windows and Frames
 
@@ -3064,7 +3088,8 @@ using the commands of Lisp mode, because they trigger this
 readjustment.  To test such code, put it into a command and bind the
 command to a key.
 
-@defun window-start &optional window
+@defun window-start &optional window group
+@vindex window-start-group-function
 @cindex window top line
 This function returns the display-start position of window
 @var{window}.  If @var{window} is @code{nil}, the selected window is
@@ -3080,10 +3105,20 @@ it explicitly since the previous redisplay)---to make sure point appears
 on the screen.  Nothing except redisplay automatically changes the
 window-start position; if you move point, do not expect the window-start
 position to change in response until after the next redisplay.
+
+If @var{group} is non-@code{nil}, and @var{window} is a part of a
+group of windows (@pxref{Basic Windows}), @code{window-start} returns
+the start position of the entire group.  This condition holds when the
+buffer local variable @code{window-start-group-function} is set to a
+function.  In this case, @code{window-start} calls the function with
+the single argument @var{window}, then returns its result, instead of
+performing the actions described above.  Typically, the function will
+call @code{window-start} recursively.
 @end defun
 
+@vindex window-end-group-function
 @cindex window end position
-@defun window-end &optional window update
+@defun window-end &optional window update group
 This function returns the position where display of its buffer ends in
 @var{window}.  The default for @var{window} is the selected window.
 
@@ -3106,9 +3141,19 @@ attempt to scroll the display if point has moved off the screen, the
 way real redisplay would do.  It does not alter the
 @code{window-start} value.  In effect, it reports where the displayed
 text will end if scrolling is not required.
+
+If @var{group} is non-@code{nil}, and @var{window} is a part of a
+group of windows (@pxref{Basic Windows}), `window-end' returns the end
+position of the entire group.  This condition holds when the buffer
+local variable @code{window-end-group-function} is set to a function.
+In this case, @code{window-end} calls the function with the two
+arguments @var{window} and @var{update}, then returns its result,
+instead of performing the actions described above.  Typically, the
+function will call @code{window-end} recursively.
 @end defun
 
-@defun set-window-start window position &optional noforce
+@vindex set-window-start-group-function
+@defun set-window-start window position &optional noforce group
 This function sets the display-start position of @var{window} to
 @var{position} in @var{window}'s buffer.  It returns @var{position}.
 
@@ -3169,9 +3214,20 @@ it is still 1 when redisplay occurs.  Here is an example:
 If @var{noforce} is non-@code{nil}, and @var{position} would place point
 off screen at the next redisplay, then redisplay computes a new window-start
 position that works well with point, and thus @var{position} is not used.
+
+If @var{group} is non-@code{nil}, and @var{window} is a part of a
+group of windows (@pxref{Basic Windows}), @code{set-window-start} sets
+the start position of the entire group.  This condition holds when the
+buffer local variable @code{set-window-start-group-function} is set to
+a function.  In this case, @code{set-window-start} calls the function
+with the three arguments @var{window}, @var{position}, and
+@var{noforce}, then returns its result, instead of performing the
+actions described above.  Typically, the function will call
+@code{set-window-start} recursively.
 @end defun
 
-@defun pos-visible-in-window-p &optional position window partially
+@defun pos-visible-in-window-p &optional position window partially group
+@vindex pos-visible-in-window-p-group-function
 This function returns non-@code{nil} if @var{position} is within the
 range of text currently visible on the screen in @var{window}.  It
 returns @code{nil} if @var{position} is scrolled vertically out of
@@ -3210,6 +3266,18 @@ Here is an example:
     (recenter 0))
 @end group
 @end example
+
+If @var{group} is non-@code{nil}, and @var{window} is a part of a
+group of windows (@pxref{Basic Windows}),
+@code{pos-visible-in-window-p} tests the visibility of @var{pos} in
+the entire group, not just in the single @var{window}.  This condition
+holds when the buffer local variable
+@code{pos-visible-in-window-p-group-function} is set to a function.
+In this case @code{pos-visible-in-window-p} calls the function with
+the three arguments @var{position}, @var{window}, and @var{partially},
+then returns its result, instead of performing the actions described
+above.  Typically, the function will call
+@code{pos-visible-in-window-p} recursively.
 @end defun
 
 @defun window-line-height &optional line window
@@ -3427,7 +3495,8 @@ beginning or end of the buffer (depending on scrolling direction);
 only if point is already on that position do they signal an error.
 @end defopt
 
-@deffn Command recenter &optional count
+@deffn Command recenter &optional count group
+@vindex recenter-group-function
 @cindex centering point
 This function scrolls the text in the selected window so that point is
 displayed at a specified vertical position within the window.  It does
@@ -3444,6 +3513,14 @@ If @var{count} is @code{nil} (or a non-@code{nil} list),
 window.  If @var{count} is @code{nil}, this function may redraw the
 frame, according to the value of @code{recenter-redisplay}.
 
+If @var{group} is non-@code{nil}, and the selected window is part of a
+group of windows (@pxref{Basic Windows}), @code{recenter} scrolls the
+entire group.  This condition holds when the buffer local variable
+@code{recenter-group-function} is set to a function.  In this case,
+@code{recenter} calls the function with the argument @var{count}, then
+returns its result, instead of performing the actions described above.
+Typically, the function will call @code{recenter} recursively.
+
 When @code{recenter} is called interactively, @var{count} is the raw
 prefix argument.  Thus, typing @kbd{C-u} as the prefix sets the
 @var{count} to a non-@code{nil} list, while typing @kbd{C-u 4} sets
diff --git a/lisp/follow.el b/lisp/follow.el
index 938c59e8506..f2427374f24 100644
--- a/lisp/follow.el
+++ b/lisp/follow.el
@@ -421,7 +421,19 @@ Keys specific to Follow mode:
       (progn
         (add-hook 'compilation-filter-hook 'follow-align-compilation-windows t t)
         (add-hook 'post-command-hook 'follow-post-command-hook t)
-        (add-hook 'window-size-change-functions 'follow-window-size-change t))
+        (add-hook 'window-size-change-functions 'follow-window-size-change t)
+        (add-hook 'after-change-functions 'follow-after-change nil t)
+
+        (setq window-start-group-function 'follow-window-start)
+        (setq window-end-group-function 'follow-window-end)
+        (setq set-window-start-group-function 'follow-set-window-start)
+        (setq recenter-group-function 'follow-recenter)
+        (setq pos-visible-in-window-p-group-function
+              'follow-pos-visible-in-window-p)
+        (setq selected-window-group-function 'follow-all-followers)
+        (setq move-to-window-line-group-function 'follow-move-to-window-line)
+        (setq sit*-for-function 'follow-sit-for))
+
     ;; Remove globally-installed hook functions only if there is no
     ;; other Follow mode buffer.
     (let ((buffers (buffer-list))
@@ -432,6 +444,17 @@ Keys specific to Follow mode:
       (unless following
         (remove-hook 'post-command-hook 'follow-post-command-hook)
         (remove-hook 'window-size-change-functions 'follow-window-size-change)))
+
+    (kill-local-variable 'sit*-for-function)
+    (kill-local-variable 'move-to-window-line-group-function)
+    (kill-local-variable 'selected-window-group-function)
+    (kill-local-variable 'pos-visible-in-window-p-group-function)
+    (kill-local-variable 'recenter-group-function)
+    (kill-local-variable 'set-window-start-group-function)
+    (kill-local-variable 'window-end-group-function)
+    (kill-local-variable 'window-start-group-function)
+
+    (remove-hook 'after-change-functions 'follow-after-change t)
     (remove-hook 'compilation-filter-hook 'follow-align-compilation-windows t)))
 
 (defun follow-find-file-hook ()
@@ -1015,6 +1038,10 @@ Otherwise, return nil."
 ;; is nil.  Start every window directly after the end of the previous
 ;; window, to make sure long lines are displayed correctly.
 
+(defvar follow-start-end-invalid t
+  "When non-nil, indicates `follow-windows-start-end-cache' is invalid.")
+(make-variable-buffer-local 'follow-start-end-invalid)
+
 (defun follow-redisplay (&optional windows win preserve-win)
   "Reposition the WINDOWS around WIN.
 Should point be too close to the roof we redisplay everything
@@ -1047,7 +1074,8 @@ repositioning the other windows."
     (dolist (w windows)
       (unless (and preserve-win (eq w win))
         (set-window-start w start))
-      (setq start (car (follow-calc-win-end w))))))
+      (setq start (car (follow-calc-win-end w))))
+    (setq follow-start-end-invalid nil)))
 
 (defun follow-estimate-first-window-start (windows win start)
   "Estimate the position of the first window.
@@ -1446,6 +1474,167 @@ non-first windows in Follow mode."
 
 (add-hook 'window-scroll-functions 'follow-avoid-tail-recenter t)
 
+;;; Low level window start and end.
+
+;; These routines are the Follow Mode versions of the low level
+;; functions described on page "Window Start and End" of the elisp
+;; manual, e.g. `window*-start'.  The aim is to be able to handle
+;; Follow Mode windows by replacing `window-start' by `window*-start',
+;; etc.
+
+(defun follow-after-change (_beg _end _old-len)
+  "After change function: set `follow-start-end-invalid'."
+  (setq follow-start-end-invalid t))
+
+(defun follow-window-start (&optional window)
+  "Return position at which display currently starts in the
+Follow Mode group of windows which includes WINDOW.
+
+WINDOW must be a live window and defaults to the selected one.
+This is updated by redisplay or by calling
+`follow-set-window-start'."
+  (let ((windows (follow-all-followers window)))
+    (window-start (car windows))))
+
+(defun follow-window-end (&optional window update)
+  "Return position at which display currently ends in the Follow
+  Mode group of windows which includes WINDOW.
+
+  WINDOW must be a live window and defaults to the selected one.
+  This is updated by redisplay, when it runs to completion.
+  Simply changing the buffer text or setting `window-start' does
+  not update this value.
+
+  Return nil if there is no recorded value.  (This can happen if
+  the last redisplay of WINDOW was preempted, and did not
+  finish.)  If UPDATE is non-nil, compute the up-to-date position
+  if it isn't already recorded."
+  (let* ((windows (follow-all-followers window))
+         (last (car (last windows))))
+    (when (and update follow-start-end-invalid)
+      (follow-redisplay windows (car windows)))
+    (window-end last update)))
+
+(defun follow-set-window-start (window pos &optional noforce)
+  "Make display in the Follow Mode group of windows which includes
+WINDOW start at position POS in WINDOW's buffer.
+
+WINDOW must be a live window and defaults to the selected one.  Return
+POS.  Optional third arg NOFORCE non-nil inhibits next redisplay from
+overriding motion of point in order to display at this exact start."
+  (let ((windows (follow-all-followers window)))
+    (setq follow-start-end-invalid t)
+    (set-window-start (car windows) pos noforce)))
+
+(defun follow-pos-visible-in-window-p (&optional pos window partially)
+  "Return non-nil if position POS is currently on the frame in one of
+  the windows in the Follow Mode group which includes WINDOW.
+
+WINDOW must be a live window and defaults to the selected one.
+
+Return nil if that position is scrolled vertically out of view.  If a
+character is only partially visible, nil is returned, unless the
+optional argument PARTIALLY is non-nil.  If POS is only out of view
+because of horizontal scrolling, return non-nil.  If POS is t, it
+specifies the position of the last visible glyph in WINDOW.  POS
+defaults to point in WINDOW; WINDOW defaults to the selected window.
+
+If POS is visible, return t if PARTIALLY is nil; if PARTIALLY is non-nil,
+the return value is a list of 2 or 6 elements (X Y [RTOP RBOT ROWH VPOS]),
+where X and Y are the pixel coordinates relative to the top left corner
+of the actual window containing it.  The remaining elements are
+omitted if the character after POS is fully visible; otherwise, RTOP
+and RBOT are the number of pixels off-window at the top and bottom of
+the screen line (\"row\") containing POS, ROWH is the visible height
+of that row, and VPOS is the row number \(zero-based)."
+  (let* ((windows (follow-all-followers window))
+         (last (car (last windows))))
+    (when follow-start-end-invalid
+      (follow-redisplay windows (car windows)))
+    (let* ((cache (follow-windows-start-end windows))
+           (last-elt (car (last cache)))
+           our-pos pertinent-elt)
+      (setq pertinent-elt
+            (if (eq pos t)
+                last-elt
+              (setq our-pos (or pos (point)))
+              (catch 'element
+                (while cache
+                  (when (< our-pos (nth 2 (car cache)))
+                    (throw 'element (car cache)))
+                  (setq cache (cdr cache)))
+                last-elt)))
+      (pos-visible-in-window-p our-pos (car pertinent-elt) partially))))
+
+(defun follow-move-to-window-line (arg)
+  "Position point relative to the Follow mode group containing the selected window.
+ARG nil means position point at center of the window group.
+Else, ARG specifies vertical position within the window group;
+zero means top of the first window in the group, negative means
+  relative to bottom of the last window in the group."
+  (let* ((windows (follow-all-followers))
+         (start-end (follow-windows-start-end windows))
+         (rev-start-end (reverse start-end))
+         (lines 0)
+         middle-window elt count)
+    (select-window
+     (cond
+      ((null arg)
+       (setq rev-start-end (nthcdr (/ (length windows) 2) rev-start-end))
+       (prog1 (car (car rev-start-end))
+         (while (setq rev-start-end (cdr rev-start-end))
+           (setq elt (car rev-start-end)
+                 count (count-screen-lines (cadr elt) (nth 2 elt)
+                                           nil (car elt))
+                 lines (+ lines count)))))
+      ((>= arg 0)
+       (while (and (cdr start-end)
+                   (progn
+                     (setq elt (car start-end)
+                           count (count-screen-lines (cadr elt) (nth 2 elt)
+                                                     nil (car elt)))
+                     (>= arg count)))
+         (setq arg (- arg count)
+               lines (+ lines count)
+               start-end (cdr start-end)))
+       (car (car start-end)))
+      (t                                ; (< arg 0)
+       (while (and (cadr rev-start-end)
+                   (progn
+                     (setq elt (car rev-start-end)
+                           count (count-lines (cadr elt) (nth 2 elt)))
+                     (<= arg (- count))))
+         (setq arg (+ arg count)
+               rev-start-end (cdr rev-start-end)))
+       (prog1 (car (car rev-start-end))
+         (while (setq rev-start-end (cdr rev-start-end))
+           (setq elt (car rev-start-end)
+                 count (count-screen-lines (cadr elt) (nth 2 elt)
+                                           nil (car elt))
+                 lines (+ lines count)))))))
+    (+ lines (move-to-window-line arg))))
+
+(defun follow-sit-for (seconds &optional nodisp)
+  "Redisplay, then wait for SECONDS seconds.  Stop when input is available.
+Before redisplaying, synchronise all Follow windows.
+
+SECONDS may be a floating-point value.
+\(On operating systems that do not support waiting for fractions of a
+second, floating-point values are rounded down to the nearest integer.)
+
+Redisplay does not happen if input is available before it starts.
+If optional arg NODISP is t, don't synchronise or redisplay, just
+wait for input.
+
+Value is t if waited the full time with no input arriving, and nil
+otherwise.
+
+The functionality is intended to be the same as `sit-for''s."
+  (when (and (not (input-pending-p t))
+             (not nodisp))
+    (follow-adjust-window (selected-window)))
+  (sit-for seconds nodisp))
+
 ;;; Profile support
 
 ;; The following (non-evaluated) section can be used to
diff --git a/lisp/isearch.el b/lisp/isearch.el
index 9f8ba8d8d7b..92d7894d2e7 100644
--- a/lisp/isearch.el
+++ b/lisp/isearch.el
@@ -171,6 +171,11 @@ is non-nil if the user quits the search.")
   "Function to call to display the search prompt.
 If nil, use function `isearch-message'.")
 
+(defmacro isearch-call-message (&optional cqh ellip)
+  `(if isearch-message-function
+       (funcall isearch-message-function ,cqh ,ellip)
+     (isearch-message ,cqh ,ellip)))
+
 (defvar isearch-wrap-function nil
   "Function to call to wrap the search when search is failed.
 If nil, move point to the beginning of the buffer for a forward search,
@@ -969,12 +974,10 @@ The last thing it does is to run `isearch-update-post-hook'."
            (null executing-kbd-macro))
       (progn
         (if (not (input-pending-p))
-            (if isearch-message-function
-                (funcall isearch-message-function)
-              (isearch-message)))
+            (isearch-call-message))
         (if (and isearch-slow-terminal-mode
                  (not (or isearch-small-window
-                          (pos-visible-in-window-p))))
+                          (pos-visible-in-window-p nil nil nil t))))
             (let ((found-point (point)))
               (setq isearch-small-window t)
               (move-to-window-line 0)
@@ -995,7 +998,7 @@ The last thing it does is to run `isearch-update-post-hook'."
           (let ((current-scroll (window-hscroll))
                 visible-p)
             (set-window-hscroll (selected-window) isearch-start-hscroll)
-            (setq visible-p (pos-visible-in-window-p nil nil t))
+            (setq visible-p (pos-visible-in-window-p nil nil t t))
             (if (or (not visible-p)
                     ;; When point is not visible because of hscroll,
                     ;; pos-visible-in-window-p returns non-nil, but
@@ -1049,7 +1052,7 @@ NOPUSH is t and EDIT is t."
   (setq minibuffer-message-timeout isearch-original-minibuffer-message-timeout)
   (isearch-dehighlight)
   (lazy-highlight-cleanup lazy-highlight-cleanup)
-  (let ((found-start (window-start))
+  (let ((found-start (window-start nil t))
         (found-point (point)))
     (when isearch-window-configuration
       (set-window-configuration isearch-window-configuration)
@@ -1059,7 +1062,7 @@ NOPUSH is t and EDIT is t."
         ;; This has an annoying side effect of clearing the last_modiff
         ;; field of the window, which can cause unwanted scrolling,
         ;; so don't do it unless truly necessary.
-        (set-window-start (selected-window) found-start t))))
+        (set-window-start (selected-window) found-start t t))))
 
   (setq isearch-mode nil)
   (if isearch-input-method-local-p
@@ -1282,13 +1285,6 @@ You can update the global isearch variables by setting new values to
           (unwind-protect
               (progn ,@body)
 
-            ;; Set point at the start (end) of old match if forward (backward),
-            ;; so after exiting minibuffer isearch resumes at the start (end)
-            ;; of this match and can find it again.
-            (if (and old-other-end (eq old-point (point))
-                     (eq isearch-forward isearch-new-forward))
-                (goto-char old-other-end))
-
             ;; Always resume isearching by restarting it.
             (isearch-mode isearch-forward
                           isearch-regexp
@@ -1301,7 +1297,17 @@ You can update the global isearch variables by setting new values to
                   isearch-message isearch-new-message
                   isearch-forward isearch-new-forward
                   isearch-regexp-function isearch-new-regexp-function
-                  isearch-case-fold-search isearch-new-case-fold))
+                  isearch-case-fold-search isearch-new-case-fold)
+
+            ;; Restore the minibuffer message before moving point.
+            (isearch-call-message nil t)
+
+            ;; Set point at the start (end) of old match if forward (backward),
+            ;; so after exiting minibuffer isearch resumes at the start (end)
+            ;; of this match and can find it again.
+            (if (and old-other-end (eq old-point (point))
+                     (eq isearch-forward isearch-new-forward))
+                (goto-char old-other-end)))
 
           ;; Empty isearch-string means use default.
           (when (= 0 (length isearch-string))
@@ -1895,6 +1901,7 @@ If search string is empty, just beep."
                                             (length isearch-string))))
           isearch-message (mapconcat 'isearch-text-char-description
                                      isearch-string "")))
+  (isearch-call-message nil t)          ; Do this before moving point.
   ;; Use the isearch-other-end as new starting point to be able
   ;; to find the remaining part of the search string again.
   ;; This is like what `isearch-search-and-update' does,
@@ -2071,6 +2078,7 @@ With argument, add COUNT copies of the character."
               (setq isearch-case-fold-search
                     (isearch-no-upper-case-p isearch-string isearch-regexp))))
       ;; Not regexp, not reverse, or no match at point.
+      (isearch-call-message nil t)      ; Do this before moving point.
       (if (and isearch-other-end (not isearch-adjusted))
           (goto-char (if isearch-forward isearch-other-end
                        (min isearch-opoint
@@ -2237,10 +2245,12 @@ Return nil if it's completely visible, or if point is visible,
 together with as much of the search string as will fit; the symbol
 `above' if we need to scroll the text downwards; the symbol `below',
 if upwards."
-  (let ((w-start (window-start))
-        (w-end (window-end nil t))
-        (w-L1 (save-excursion (move-to-window-line 1) (point)))
-        (w-L-1 (save-excursion (move-to-window-line -1) (point)))
+  (let ((w-start (window-start nil t))
+        (w-end (window-end nil t t))
+        (w-L1 (save-excursion
+                (save-selected-window (move-to-window-line 1 t) (point))))
+        (w-L-1 (save-excursion
+                 (save-selected-window (move-to-window-line -1 t) (point))))
         start end)                  ; start and end of search string in buffer
     (if isearch-forward
         (setq end isearch-point  start (or isearch-other-end isearch-point))
@@ -2267,15 +2277,15 @@ the bottom."
     (if above
         (progn
           (goto-char start)
-          (recenter 0)
-          (when (>= isearch-point (window-end nil t))
+          (recenter 0 t)
+          (when (>= isearch-point (window-end nil t t))
             (goto-char isearch-point)
-            (recenter -1)))
+            (recenter -1 t)))
       (goto-char end)
-      (recenter -1)
-      (when (< isearch-point (window-start))
+      (recenter -1 t)
+      (when (< isearch-point (window-start nil t))
         (goto-char isearch-point)
-        (recenter 0))))
+        (recenter 0 t))))
   (goto-char isearch-point))
 
 (defvar isearch-pre-scroll-point nil)
@@ -2422,6 +2432,7 @@ Search is updated accordingly."
   (isearch-ring-adjust1 advance)
   (if search-ring-update
       (progn
+        (isearch-call-message nil t)
         (isearch-search)
         (isearch-push-state)
         (isearch-update))
@@ -2494,6 +2505,13 @@ If there is no completion possible, say so and continue searching."
 
 (defun isearch-message (&optional c-q-hack ellipsis)
   ;; Generate and print the message string.
+
+  ;; N.B.: This function should always be called with point at the
+  ;; search point, because in certain (rare) circumstances, undesired
+  ;; scrolling can happen when point is elsewhere.  These
+  ;; circumstances are when follow-mode is active, the search string
+  ;; spans two (or several) windows, and the message about to be
+  ;; displayed will cause the echo area to expand.
   (let ((cursor-in-echo-area ellipsis)
         (m isearch-message)
         (fail-pos (isearch-fail-pos t)))
@@ -2680,9 +2698,6 @@ update the match data, and return point."
 
 (defun isearch-search ()
   ;; Do the search with the current search string.
-  (if isearch-message-function
-      (funcall isearch-message-function nil t)
-    (isearch-message nil t))
   (if (and (eq isearch-case-fold-search t) search-upper-case)
       (setq isearch-case-fold-search
             (isearch-no-upper-case-p isearch-string isearch-regexp)))
@@ -2980,6 +2995,7 @@ since they have special meaning in a regexp."
 (defvar isearch-lazy-highlight-timer nil)
 (defvar isearch-lazy-highlight-last-string nil)
 (defvar isearch-lazy-highlight-window nil)
+(defvar isearch-lazy-highlight-window-group nil)
 (defvar isearch-lazy-highlight-window-start nil)
 (defvar isearch-lazy-highlight-window-end nil)
 (defvar isearch-lazy-highlight-case-fold-search nil)
@@ -3012,7 +3028,21 @@ is nil.  This function is called when exiting an incremental search if
                                 "22.1")
 
 (defun isearch-lazy-highlight-new-loop (&optional beg end)
-  "Cleanup any previous `lazy-highlight' loop and begin a new one.
+  "Set an idle timer, which will trigger a new `lazy-highlight' loop.
+BEG and END specify the bounds within which highlighting should
+occur.  This is called when `isearch-update' is invoked (which
+can cause the search string to change or the window(s) to
+scroll).  It is also used by other Emacs features.  Do not start
+the loop when we are executing a keyboard macro."
+  (setq isearch-lazy-highlight-start-limit beg
+        isearch-lazy-highlight-end-limit end)
+  (when (null executing-kbd-macro)
+    (setq isearch-lazy-highlight-timer
+          (run-with-idle-timer lazy-highlight-initial-delay nil
+                               'isearch-lazy-highlight-maybe-new-loop))))
+
+(defun isearch-lazy-highlight-maybe-new-loop ()
+  "If needed cleanup any previous `lazy-highlight' loop and begin a new one.
 BEG and END specify the bounds within which highlighting should occur.
 This is called when `isearch-update' is invoked (which can cause the
 search string to change or the window to scroll).  It is also used
@@ -3021,8 +3051,8 @@ by other Emacs features."
              (sit-for 0)         ;make sure (window-start) is credible
              (or (not (equal isearch-string
                              isearch-lazy-highlight-last-string))
-                 (not (eq (selected-window)
-                          isearch-lazy-highlight-window))
+                 (not (memq (selected-window)
+                            isearch-lazy-highlight-window-group))
                  (not (eq isearch-lazy-highlight-case-fold-search
                           isearch-case-fold-search))
                  (not (eq isearch-lazy-highlight-regexp
@@ -3033,9 +3063,9 @@ by other Emacs features."
                           isearch-lax-whitespace))
                  (not (eq isearch-lazy-highlight-regexp-lax-whitespace
                           isearch-regexp-lax-whitespace))
-                 (not (= (window-start)
+                 (not (= (window-start nil t)
                          isearch-lazy-highlight-window-start))
-                 (not (= (window-end)   ; Window may have been split/joined.
+                 (not (= (window-end nil nil t) ; Window may have been split/joined.
                          isearch-lazy-highlight-window-end))
                  (not (eq isearch-forward
                           isearch-lazy-highlight-forward))
@@ -3043,16 +3073,15 @@ by other Emacs features."
                  (not (equal isearch-error
                              isearch-lazy-highlight-error))))
     ;; something important did indeed change
-    (lazy-highlight-cleanup t) ;kill old loop & remove overlays
+    (lazy-highlight-cleanup t)        ;kill old loop & remove overlays
     (setq isearch-lazy-highlight-error isearch-error)
     ;; It used to check for `(not isearch-error)' here, but actually
     ;; lazy-highlighting might find matches to highlight even when
     ;; `isearch-error' is non-nil.  (Bug#9918)
-    (setq isearch-lazy-highlight-start-limit beg
-          isearch-lazy-highlight-end-limit end)
     (setq isearch-lazy-highlight-window       (selected-window)
-          isearch-lazy-highlight-window-start (window-start)
-          isearch-lazy-highlight-window-end   (window-end)
+          isearch-lazy-highlight-window-group (selected-window-group)
+          isearch-lazy-highlight-window-start (window-start nil t)
+          isearch-lazy-highlight-window-end   (window-end nil nil t)
           ;; Start lazy-highlighting at the beginning of the found
           ;; match (`isearch-other-end').  If no match, use point.
           ;; One of the next two variables (depending on search direction)
@@ -3070,10 +3099,8 @@ by other Emacs features."
           isearch-lazy-highlight-regexp-lax-whitespace isearch-regexp-lax-whitespace
           isearch-lazy-highlight-regexp-function  isearch-regexp-function
           isearch-lazy-highlight-forward      isearch-forward)
-      (unless (equal isearch-string "")
-        (setq isearch-lazy-highlight-timer
-              (run-with-idle-timer lazy-highlight-initial-delay nil
-                                   'isearch-lazy-highlight-update)))))
+    (unless (equal isearch-string "")
+      (isearch-lazy-highlight-update))))
 
 (defun isearch-lazy-highlight-search ()
   "Search ahead for the next or previous match, for lazy highlighting.
@@ -3096,13 +3123,13 @@ Attempt to do the search exactly the way the pending Isearch would."
                                 (+ isearch-lazy-highlight-start
                                    ;; Extend bound to match whole string at point
                                    (1- (length isearch-lazy-highlight-last-string)))
-                              (window-end)))
+                              (window-end nil nil t)))
                      (max (or isearch-lazy-highlight-start-limit (point-min))
                           (if isearch-lazy-highlight-wrapped
                               (- isearch-lazy-highlight-end
                                  ;; Extend bound to match whole string at point
                                  (1- (length isearch-lazy-highlight-last-string)))
-                            (window-start))))))
+                            (window-start nil t))))))
         ;; Use a loop like in `isearch-search'.
         (while retry
           (setq success (isearch-search-string
@@ -3126,7 +3153,7 @@ Attempt to do the search exactly the way the pending Isearch would."
     (with-local-quit
       (save-selected-window
         (if (and (window-live-p isearch-lazy-highlight-window)
-                 (not (eq (selected-window) isearch-lazy-highlight-window)))
+                 (not (memq (selected-window) isearch-lazy-highlight-window-group)))
             (select-window isearch-lazy-highlight-window))
         (save-excursion
           (save-match-data
@@ -3146,12 +3173,12 @@ Attempt to do the search exactly the way the pending Isearch would."
                           (if isearch-lazy-highlight-forward
                               (if (= mb (if isearch-lazy-highlight-wrapped
                                             isearch-lazy-highlight-start
-                                          (window-end)))
+                                          (window-end nil nil t)))
                                   (setq found nil)
                                 (forward-char 1))
                             (if (= mb (if isearch-lazy-highlight-wrapped
                                           isearch-lazy-highlight-end
-                                        (window-start)))
+                                        (window-start nil t)))
                                 (setq found nil)
                               (forward-char -1)))
 
@@ -3161,8 +3188,8 @@ Attempt to do the search exactly the way the pending Isearch would."
                           ;; 1000 is higher than ediff's 100+,
                           ;; 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))))
+                          (overlay-put ov 'face lazy-highlight-face)))
+                          ;(overlay-put ov 'window (selected-window))))
                       ;; Remember the current position of point for
                       ;; the next call of `isearch-lazy-highlight-update'
                       ;; when `lazy-highlight-max-at-a-time' is too small.
@@ -3178,12 +3205,12 @@ Attempt to do the search exactly the way the pending Isearch would."
                       (setq isearch-lazy-highlight-wrapped t)
                       (if isearch-lazy-highlight-forward
                           (progn
-                            (setq isearch-lazy-highlight-end (window-start))
+                            (setq isearch-lazy-highlight-end (window-start nil t))
                             (goto-char (max (or isearch-lazy-highlight-start-limit (point-min))
-                                            (window-start))))
-                        (setq isearch-lazy-highlight-start (window-end))
+                                            (window-start nil t))))
+                        (setq isearch-lazy-highlight-start (window-end nil nil t))
                         (goto-char (min (or isearch-lazy-highlight-end-limit (point-max))
-                                        (window-end))))))))
+                                        (window-end nil nil t))))))))
             (unless nomore
               (setq isearch-lazy-highlight-timer
                     (run-at-time lazy-highlight-interval nil
diff --git a/lisp/window.el b/lisp/window.el
index 6d189055c15..0c403eb184a 100644
--- a/lisp/window.el
+++ b/lisp/window.el
@@ -28,6 +28,17 @@
 
 ;;; Code:
 
+(defvar selected-window-group-function nil)
+(make-variable-buffer-local 'selected-window-group-function)
+(put 'selected-window-group-function 'permanent-local t)
+(defun selected-window-group ()
+  "Return the list of windows in the group containing the selected window.
+When a grouping mode (such as Follow Mode) is not active, the
+result is a list containing only the selected window."
+  (if (functionp selected-window-group-function)
+      (funcall selected-window-group-function)
+    (list (selected-window))))
+
 (defun internal--before-save-selected-window ()
   (cons (selected-window)
         ;; We save and restore all frames' selected windows, because
diff --git a/src/keyboard.c b/src/keyboard.c
index 851207874db..c7733568e7f 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -10663,7 +10663,7 @@ The `posn-' functions access elements of such lists.  */)
   if (NILP (window))
     window = selected_window;
 
-  tem = Fpos_visible_in_window_p (pos, window, Qt);
+  tem = Fpos_visible_in_window_p (pos, window, Qt, Qnil);
   if (!NILP (tem))
     {
       Lisp_Object x = XCAR (tem);
diff --git a/src/window.c b/src/window.c
index 0ac76d41861..efb4c9b4f8c 100644
--- a/src/window.c
+++ b/src/window.c
@@ -1540,13 +1540,23 @@ WINDOW must be a live window and defaults to the selected one.  */)
   return Fmarker_position (decode_live_window (window)->old_pointm);
 }
 
-DEFUN ("window-start", Fwindow_start, Swindow_start, 0, 1, 0,
+DEFUN ("window-start", Fwindow_start, Swindow_start, 0, 2, 0,
        doc: /* Return position at which display currently starts in WINDOW.
 WINDOW must be a live window and defaults to the selected one.
-This is updated by redisplay or by calling `set-window-start'.  */)
-  (Lisp_Object window)
+This is updated by redisplay or by calling `set-window-start'.
+
+If GROUP is non-nil, and WINDOW is part of a group of windows collectively
+displaying a buffer (such as with Follow Mode), return the start position of
+the group rather than of the individual WINDOW.  This condition holds when
+`window-start-group-function' is set to a function, in which case
+`window-start' calls the function with the argument WINDOW, then returns its
+result, instead of doing its normal processing.  */)
+  (Lisp_Object window, Lisp_Object group)
 {
-  return Fmarker_position (decode_live_window (window)->start);
+  return (!NILP (group)
+          && FUNCTIONP (Vwindow_start_group_function))
+    ? call1 (Vwindow_start_group_function, window)
+    : Fmarker_position (decode_live_window (window)->start);
 }
 
 /* This is text temporarily removed from the doc string below.
@@ -1560,7 +1570,7 @@ have been if redisplay had finished, do this:
       (vertical-motion (1- (window-height window)) window)
       (point))")  */
 
-DEFUN ("window-end", Fwindow_end, Swindow_end, 0, 2, 0,
+DEFUN ("window-end", Fwindow_end, Swindow_end, 0, 3, 0,
        doc: /* Return position at which display currently ends in WINDOW.
 WINDOW must be a live window and defaults to the selected one.
 This is updated by redisplay, when it runs to completion.
@@ -1569,65 +1579,77 @@ does not update this value.
 Return nil if there is no recorded value.  (This can happen if the
 last redisplay of WINDOW was preempted, and did not finish.)
 If UPDATE is non-nil, compute the up-to-date position
-if it isn't already recorded.  */)
-  (Lisp_Object window, Lisp_Object update)
-{
-  Lisp_Object value;
-  struct window *w = decode_live_window (window);
-  Lisp_Object buf;
-  struct buffer *b;
-
-  buf = w->contents;
-  CHECK_BUFFER (buf);
-  b = XBUFFER (buf);
-
-  if (! NILP (update)
-      && (windows_or_buffers_changed
-          || !w->window_end_valid
-          || b->clip_changed
-          || b->prevent_redisplay_optimizations_p
-          || window_outdated (w))
-      /* Don't call display routines if we didn't yet create any real
-         frames, because the glyph matrices are not yet allocated in
-         that case.  This could happen in some code that runs in the
-         daemon during initialization (e.g., see bug#20565).  */
-      && !(noninteractive || FRAME_INITIAL_P (WINDOW_XFRAME (w))))
-    {
-      struct text_pos startp;
-      struct it it;
-      struct buffer *old_buffer = NULL;
-      void *itdata = NULL;
-
-      /* Cannot use Fvertical_motion because that function doesn't
-         cope with variable-height lines.  */
-      if (b != current_buffer)
-        {
-          old_buffer = current_buffer;
-          set_buffer_internal (b);
-        }
-
-      /* In case W->start is out of the range, use something
-         reasonable.  This situation occurred when loading a file with
-         `-l' containing a call to `rmail' with subsequent other
-         commands.  At the end, W->start happened to be BEG, while
-         rmail had already narrowed the buffer.  */
-      CLIP_TEXT_POS_FROM_MARKER (startp, w->start);
-
-      itdata = bidi_shelve_cache ();
-      start_display (&it, w, startp);
-      move_it_vertically (&it, window_box_height (w));
-      if (it.current_y < it.last_visible_y)
-        move_it_past_eol (&it);
-      value = make_number (IT_CHARPOS (it));
-      bidi_unshelve_cache (itdata, false);
-
-      if (old_buffer)
-        set_buffer_internal (old_buffer);
-    }
-  else
-    XSETINT (value, BUF_Z (b) - w->window_end_pos);
+if it isn't already recorded.
+
+If GROUP is non-nil, and WINDOW is part of a group of windows collectively
+displaying a buffer (such as with Follow Mode), return the end position of
+the group rather than of the individual WINDOW.  This condition holds when
+`window-end-group-function' is set to a function, in which case `window-end'
+calls the function with the two arguments WINDOW and UPDATE, then returns its
+result, instead of doing its normal processing.  */)
+  (Lisp_Object window, Lisp_Object update, Lisp_Object group)
+{
+  if (!NILP (group)
+      && FUNCTIONP (Vwindow_end_group_function))
+    return call2 (Vwindow_end_group_function, window, update);
+  {
+    Lisp_Object value;
+    struct window *w = decode_live_window (window);
+    Lisp_Object buf;
+    struct buffer *b;
+
+    buf = w->contents;
+    CHECK_BUFFER (buf);
+    b = XBUFFER (buf);
+
+    if (! NILP (update)
+        && (windows_or_buffers_changed
+            || !w->window_end_valid
+            || b->clip_changed
+            || b->prevent_redisplay_optimizations_p
+            || window_outdated (w))
+        /* Don't call display routines if we didn't yet create any real
+           frames, because the glyph matrices are not yet allocated in
+           that case.  This could happen in some code that runs in the
+           daemon during initialization (e.g., see bug#20565).  */
+        && !(noninteractive || FRAME_INITIAL_P (WINDOW_XFRAME (w))))
+      {
+        struct text_pos startp;
+        struct it it;
+        struct buffer *old_buffer = NULL;
+        void *itdata = NULL;
+
+        /* Cannot use Fvertical_motion because that function doesn't
+           cope with variable-height lines.  */
+        if (b != current_buffer)
+          {
+            old_buffer = current_buffer;
+            set_buffer_internal (b);
+          }
+
+        /* In case W->start is out of the range, use something
+           reasonable.  This situation occurred when loading a file with
+           `-l' containing a call to `rmail' with subsequent other
+           commands.  At the end, W->start happened to be BEG, while
+           rmail had already narrowed the buffer.  */
+        CLIP_TEXT_POS_FROM_MARKER (startp, w->start);
+
+        itdata = bidi_shelve_cache ();
+        start_display (&it, w, startp);
+        move_it_vertically (&it, window_box_height (w));
+        if (it.current_y < it.last_visible_y)
+          move_it_past_eol (&it);
+        value = make_number (IT_CHARPOS (it));
+        bidi_unshelve_cache (itdata, false);
+
+        if (old_buffer)
+          set_buffer_internal (old_buffer);
+      }
+    else
+      XSETINT (value, BUF_Z (b) - w->window_end_pos);
 
-  return value;
+    return value;
+  }
 }
 
 DEFUN ("set-window-point", Fset_window_point, Sset_window_point, 2, 2, 0,
@@ -1666,30 +1688,43 @@ Return POS.  */)
   return pos;
 }
 
-DEFUN ("set-window-start", Fset_window_start, Sset_window_start, 2, 3, 0,
+DEFUN ("set-window-start", Fset_window_start, Sset_window_start, 2, 4, 0,
        doc: /* Make display in WINDOW start at position POS in WINDOW's buffer.
 WINDOW must be a live window and defaults to the selected one.  Return
 POS.  Optional third arg NOFORCE non-nil inhibits next redisplay from
-overriding motion of point in order to display at this exact start.  */)
-  (Lisp_Object window, Lisp_Object pos, Lisp_Object noforce)
-{
-  register struct window *w = decode_live_window (window);
+overriding motion of point in order to display at this exact start.
+
+If GROUP is non-nil, and WINDOW is part of a group of windows collectively
+displaying a buffer (such as with Follow Mode), set the start position of
+the group rather than of the individual WINDOW.  This condition holds when
+`set-window-start-group-function' is set to a function, in which case
+`set-window-start' calls the function with the three arguments WINDOW, POS,
+and NOFORCE, then returns its result, instead of doing its normal
+processing.  */)
+  (Lisp_Object window, Lisp_Object pos, Lisp_Object noforce, Lisp_Object group)
+{
+  if (!NILP (group)
+      && FUNCTIONP (Vset_window_start_group_function))
+    return call3 (Vset_window_start_group_function, window, pos, noforce);
+  {
+    register struct window *w = decode_live_window (window);
 
-  set_marker_restricted (w->start, pos, w->contents);
-  /* This is not right, but much easier than doing what is right.  */
-  w->start_at_line_beg = false;
-  if (NILP (noforce))
-    w->force_start = true;
-  wset_update_mode_line (w);
-  /* Bug#15957.  */
-  w->window_end_valid = false;
-  wset_redisplay (w);
+    set_marker_restricted (w->start, pos, w->contents);
+    /* This is not right, but much easier than doing what is right.  */
+    w->start_at_line_beg = false;
+    if (NILP (noforce))
+      w->force_start = true;
+    wset_update_mode_line (w);
+    /* Bug#15957.  */
+    w->window_end_valid = false;
+    wset_redisplay (w);
 
-  return pos;
+    return pos;
+  }
 }
 
 DEFUN ("pos-visible-in-window-p", Fpos_visible_in_window_p,
-       Spos_visible_in_window_p, 0, 3, 0,
+       Spos_visible_in_window_p, 0, 4, 0,
        doc: /* Return non-nil if position POS is currently on the frame in WINDOW.
 WINDOW must be a live window and defaults to the selected one.
 
@@ -1709,9 +1744,21 @@ of the window.  The remaining elements are omitted if the character after
 POS is fully visible; otherwise, RTOP and RBOT are the number of pixels
 off-window at the top and bottom of the screen line ("row") containing
 POS, ROWH is the visible height of that row, and VPOS is the row number
-(zero-based).  */)
-  (Lisp_Object pos, Lisp_Object window, Lisp_Object partially)
-{
+(zero-based).
+
+If GROUP is non-nil, and WINDOW is part of a group of windows collectively
+displaying a buffer (such as with Follow Mode), test whether POS is visible
+in the group of windows rather than in the individual WINDOW.  This
+condition holds when `pos-visible-in-window-p-function' is set to a
+function, in which case `pos-visible-in-window-p' calls the function with
+the three arguments POS, WINDOW, and PARTIALLY, then returns its result,
+instead of doing its normal processing.  */)
+  (Lisp_Object pos, Lisp_Object window, Lisp_Object partially, Lisp_Object group)
+{
+  if (!NILP (group)
+      && FUNCTIONP (Vpos_visible_in_window_p_group_function))
+    return call3 (Vpos_visible_in_window_p_group_function, pos, window, partially);
+  {
   struct window *w;
   EMACS_INT posint;
   struct buffer *buf;
@@ -1760,6 +1807,7 @@ POS, ROWH is the visible height of that row, and VPOS is the row number
     }
 
   return in_window;
+  }
 }
 
 DEFUN ("window-line-height", Fwindow_line_height,
@@ -5185,7 +5233,7 @@ window_scroll_line_based (Lisp_Object window, int n, bool whole, bool noerror)
     }
 
   XSETFASTINT (tem, PT);
-  tem = Fpos_visible_in_window_p (tem, window, Qnil);
+  tem = Fpos_visible_in_window_p (tem, window, Qnil, Qnil);
 
   if (NILP (tem))
     {
@@ -5574,7 +5622,7 @@ displayed_window_lines (struct window *w)
 }
 
 
-DEFUN ("recenter", Frecenter, Srecenter, 0, 1, "P",
+DEFUN ("recenter", Frecenter, Srecenter, 0, 2, "P\ni",
        doc: /* Center point in selected window and maybe redisplay frame.
 With a numeric prefix argument ARG, recenter putting point on screen line ARG
 relative to the selected window.  If ARG is negative, it counts up from the
@@ -5588,208 +5636,221 @@ height needed); if `recenter-redisplay' has the special value `tty',
 then only tty frames are redrawn.
 
 Just C-u as prefix means put point in the center of the window
-and redisplay normally--don't erase and redraw the frame.  */)
-  (register Lisp_Object arg)
-{
-  struct window *w = XWINDOW (selected_window);
-  struct buffer *buf = XBUFFER (w->contents);
-  bool center_p = false;
-  ptrdiff_t charpos, bytepos;
-  EMACS_INT iarg IF_LINT (= 0);
-  int this_scroll_margin;
-
-  if (buf != current_buffer)
-    error ("`recenter'ing a window that does not display current-buffer.");
+and redisplay normally--don't erase and redraw the frame.
+
+When `recenter' is called from a program, GROUP is non-nil, and WINDOW is
+part of a group of windows collectively displaying a buffer (such as with
+Follow Mode), perform `recenter''s actions on the group rather than on the
+individual WINDOW.  This condition holds when `recenter-group-function' is
+set to a function, in which case `recenter' calls the function with the
+argument ARG, then returns its value, instead of doing its normal
+processing.  */)
+  (register Lisp_Object arg, Lisp_Object group)
+{
+  if (!NILP (group)
+      && FUNCTIONP (Vrecenter_group_function))
+    return call1 (Vrecenter_group_function, arg);
+  {
+    struct window *w = XWINDOW (selected_window);
+    struct buffer *buf = XBUFFER (w->contents);
+    bool center_p = false;
+    ptrdiff_t charpos, bytepos;
+    EMACS_INT iarg IF_LINT (= 0);
+    int this_scroll_margin;
 
-  /* If redisplay is suppressed due to an error, try again.  */
-  buf->display_error_modiff = 0;
+    if (buf != current_buffer)
+      error ("`recenter'ing a window that does not display current-buffer.");
 
-  if (NILP (arg))
-    {
-      if (!NILP (Vrecenter_redisplay)
-          && (!EQ (Vrecenter_redisplay, Qtty)
-              || !NILP (Ftty_type (selected_frame))))
-        {
-          ptrdiff_t i;
+    /* If redisplay is suppressed due to an error, try again.  */
+    buf->display_error_modiff = 0;
 
-          /* Invalidate pixel data calculated for all compositions.  */
-          for (i = 0; i < n_compositions; i++)
-            composition_table[i]->font = NULL;
+    if (NILP (arg))
+      {
+        if (!NILP (Vrecenter_redisplay)
+            && (!EQ (Vrecenter_redisplay, Qtty)
+                || !NILP (Ftty_type (selected_frame))))
+          {
+            ptrdiff_t i;
+
+            /* Invalidate pixel data calculated for all compositions.  */
+            for (i = 0; i < n_compositions; i++)
+              composition_table[i]->font = NULL;
 #if defined (HAVE_WINDOW_SYSTEM) && ! defined (USE_GTK) && ! defined (HAVE_NS)
-          WINDOW_XFRAME (w)->minimize_tool_bar_window_p = 1;
+            WINDOW_XFRAME (w)->minimize_tool_bar_window_p = 1;
 #endif
-          Fredraw_frame (WINDOW_FRAME (w));
-          SET_FRAME_GARBAGED (WINDOW_XFRAME (w));
-        }
+            Fredraw_frame (WINDOW_FRAME (w));
+            SET_FRAME_GARBAGED (WINDOW_XFRAME (w));
+          }
 
+        center_p = true;
+      }
+    else if (CONSP (arg)) /* Just C-u.  */
       center_p = true;
-    }
-  else if (CONSP (arg)) /* Just C-u.  */
-    center_p = true;
-  else
-    {
-      arg = Fprefix_numeric_value (arg);
-      CHECK_NUMBER (arg);
-      iarg = XINT (arg);
-    }
-
-  /* Do this after making BUF current
-     in case scroll_margin is buffer-local.  */
-  this_scroll_margin
-    = max (0, min (scroll_margin, w->total_lines / 4));
-
-  /* Don't use redisplay code for initial frames, as the necessary
-     data structures might not be set up yet then.  */
-  if (!FRAME_INITIAL_P (XFRAME (w->frame)))
-    {
-      if (center_p)
-        {
-          struct it it;
-          struct text_pos pt;
-          void *itdata = bidi_shelve_cache ();
-
-          SET_TEXT_POS (pt, PT, PT_BYTE);
-          start_display (&it, w, pt);
-          move_it_vertically_backward (&it, window_box_height (w) / 2);
-          charpos = IT_CHARPOS (it);
-          bytepos = IT_BYTEPOS (it);
-          bidi_unshelve_cache (itdata, false);
-        }
-      else if (iarg < 0)
-        {
-          struct it it;
-          struct text_pos pt;
-          ptrdiff_t nlines = min (PTRDIFF_MAX, -iarg);
-          int extra_line_spacing;
-          int h = window_box_height (w);
-          int ht = window_internal_height (w);
-          void *itdata = bidi_shelve_cache ();
-
-          nlines = clip_to_bounds (this_scroll_margin + 1, nlines,
-                                   ht - this_scroll_margin);
-
-          SET_TEXT_POS (pt, PT, PT_BYTE);
-          start_display (&it, w, pt);
-
-          /* Be sure we have the exact height of the full line containing PT.  */
-          move_it_by_lines (&it, 0);
-
-          /* The amount of pixels we have to move back is the window
-             height minus what's displayed in the line containing PT,
-             and the lines below.  */
-          it.current_y = 0;
-          it.vpos = 0;
-          move_it_by_lines (&it, nlines);
-
-          if (it.vpos == nlines)
-            h -= it.current_y;
-          else
-            {
-              /* Last line has no newline.  */
-              h -= line_bottom_y (&it);
-              it.vpos++;
-            }
-
-          /* Don't reserve space for extra line spacing of last line.  */
-          extra_line_spacing = it.max_extra_line_spacing;
-
-          /* If we can't move down NLINES lines because we hit
-             the end of the buffer, count in some empty lines.  */
-          if (it.vpos < nlines)
-            {
-              nlines -= it.vpos;
-              extra_line_spacing = it.extra_line_spacing;
-              h -= nlines * (FRAME_LINE_HEIGHT (it.f) + extra_line_spacing);
-            }
-          if (h <= 0)
-            {
-              bidi_unshelve_cache (itdata, false);
-              return Qnil;
-            }
-
-          /* Now find the new top line (starting position) of the window.  */
-          start_display (&it, w, pt);
-          it.current_y = 0;
-          move_it_vertically_backward (&it, h);
-
-          /* If extra line spacing is present, we may move too far
-             back.  This causes the last line to be only partially
-             visible (which triggers redisplay to recenter that line
-             in the middle), so move forward.
-             But ignore extra line spacing on last line, as it is not
-             considered to be part of the visible height of the line.
-          */
-          h += extra_line_spacing;
-          while (-it.current_y > h)
-            move_it_by_lines (&it, 1);
-
-          charpos = IT_CHARPOS (it);
-          bytepos = IT_BYTEPOS (it);
-
-          bidi_unshelve_cache (itdata, false);
-        }
-      else
-        {
-          struct it it;
-          struct text_pos pt;
-          ptrdiff_t nlines = min (PTRDIFF_MAX, iarg);
-          int ht = window_internal_height (w);
-          void *itdata = bidi_shelve_cache ();
-
-          nlines = clip_to_bounds (this_scroll_margin, nlines,
-                                   ht - this_scroll_margin - 1);
-
-          SET_TEXT_POS (pt, PT, PT_BYTE);
-          start_display (&it, w, pt);
-
-          /* Move to the beginning of screen line containing PT.  */
-          move_it_by_lines (&it, 0);
-
-          /* Move back to find the point which is ARG screen lines above PT.  */
-          if (nlines > 0)
-            {
-              it.current_y = 0;
-              it.vpos = 0;
-              move_it_by_lines (&it, -nlines);
-            }
+    else
+      {
+        arg = Fprefix_numeric_value (arg);
+        CHECK_NUMBER (arg);
+        iarg = XINT (arg);
+      }
 
-          charpos = IT_CHARPOS (it);
-          bytepos = IT_BYTEPOS (it);
+    /* Do this after making BUF current
+       in case scroll_margin is buffer-local.  */
+    this_scroll_margin
+      = max (0, min (scroll_margin, w->total_lines / 4));
 
-          bidi_unshelve_cache (itdata, false);
-        }
-    }
-  else
-    {
-      struct position pos;
-      int ht = window_internal_height (w);
+    /* Don't use redisplay code for initial frames, as the necessary
+       data structures might not be set up yet then.  */
+    if (!FRAME_INITIAL_P (XFRAME (w->frame)))
+      {
+        if (center_p)
+          {
+            struct it it;
+            struct text_pos pt;
+            void *itdata = bidi_shelve_cache ();
+
+            SET_TEXT_POS (pt, PT, PT_BYTE);
+            start_display (&it, w, pt);
+            move_it_vertically_backward (&it, window_box_height (w) / 2);
+            charpos = IT_CHARPOS (it);
+            bytepos = IT_BYTEPOS (it);
+            bidi_unshelve_cache (itdata, false);
+          }
+        else if (iarg < 0)
+          {
+            struct it it;
+            struct text_pos pt;
+            ptrdiff_t nlines = min (PTRDIFF_MAX, -iarg);
+            int extra_line_spacing;
+            int h = window_box_height (w);
+            int ht = window_internal_height (w);
+            void *itdata = bidi_shelve_cache ();
+
+            nlines = clip_to_bounds (this_scroll_margin + 1, nlines,
+                                     ht - this_scroll_margin);
+
+            SET_TEXT_POS (pt, PT, PT_BYTE);
+            start_display (&it, w, pt);
+
+            /* Be sure we have the exact height of the full line containing PT.  */
+            move_it_by_lines (&it, 0);
+
+            /* The amount of pixels we have to move back is the window
+               height minus what's displayed in the line containing PT,
+               and the lines below.  */
+            it.current_y = 0;
+            it.vpos = 0;
+            move_it_by_lines (&it, nlines);
+
+            if (it.vpos == nlines)
+              h -= it.current_y;
+            else
+              {
+                /* Last line has no newline.  */
+                h -= line_bottom_y (&it);
+                it.vpos++;
+              }
+
+            /* Don't reserve space for extra line spacing of last line.  */
+            extra_line_spacing = it.max_extra_line_spacing;
+
+            /* If we can't move down NLINES lines because we hit
+               the end of the buffer, count in some empty lines.  */
+            if (it.vpos < nlines)
+              {
+                nlines -= it.vpos;
+                extra_line_spacing = it.extra_line_spacing;
+                h -= nlines * (FRAME_LINE_HEIGHT (it.f) + extra_line_spacing);
+              }
+            if (h <= 0)
+              {
+                bidi_unshelve_cache (itdata, false);
+                return Qnil;
+              }
+
+            /* Now find the new top line (starting position) of the window.  */
+            start_display (&it, w, pt);
+            it.current_y = 0;
+            move_it_vertically_backward (&it, h);
+
+            /* If extra line spacing is present, we may move too far
+               back.  This causes the last line to be only partially
+               visible (which triggers redisplay to recenter that line
+               in the middle), so move forward.
+               But ignore extra line spacing on last line, as it is not
+               considered to be part of the visible height of the line.
+            */
+            h += extra_line_spacing;
+            while (-it.current_y > h)
+              move_it_by_lines (&it, 1);
+
+            charpos = IT_CHARPOS (it);
+            bytepos = IT_BYTEPOS (it);
+
+            bidi_unshelve_cache (itdata, false);
+          }
+        else
+          {
+            struct it it;
+            struct text_pos pt;
+            ptrdiff_t nlines = min (PTRDIFF_MAX, iarg);
+            int ht = window_internal_height (w);
+            void *itdata = bidi_shelve_cache ();
+
+            nlines = clip_to_bounds (this_scroll_margin, nlines,
+                                     ht - this_scroll_margin - 1);
+
+            SET_TEXT_POS (pt, PT, PT_BYTE);
+            start_display (&it, w, pt);
+
+            /* Move to the beginning of screen line containing PT.  */
+            move_it_by_lines (&it, 0);
+
+            /* Move back to find the point which is ARG screen lines above PT.  */
+            if (nlines > 0)
+              {
+                it.current_y = 0;
+                it.vpos = 0;
+                move_it_by_lines (&it, -nlines);
+              }
+
+            charpos = IT_CHARPOS (it);
+            bytepos = IT_BYTEPOS (it);
+
+            bidi_unshelve_cache (itdata, false);
+          }
+      }
+    else
+      {
+        struct position pos;
+        int ht = window_internal_height (w);
 
-      if (center_p)
-        iarg = ht / 2;
-      else if (iarg < 0)
-        iarg += ht;
+        if (center_p)
+          iarg = ht / 2;
+        else if (iarg < 0)
+          iarg += ht;
 
-      /* Don't let it get into the margin at either top or bottom.  */
-      iarg = clip_to_bounds (this_scroll_margin, iarg,
-                             ht - this_scroll_margin - 1);
+        /* Don't let it get into the margin at either top or bottom.  */
+        iarg = clip_to_bounds (this_scroll_margin, iarg,
+                               ht - this_scroll_margin - 1);
 
-      pos = *vmotion (PT, PT_BYTE, - iarg, w);
-      charpos = pos.bufpos;
-      bytepos = pos.bytepos;
-    }
+        pos = *vmotion (PT, PT_BYTE, - iarg, w);
+        charpos = pos.bufpos;
+        bytepos = pos.bytepos;
+      }
 
-  /* Set the new window start.  */
-  set_marker_both (w->start, w->contents, charpos, bytepos);
-  w->window_end_valid = false;
+    /* Set the new window start.  */
+    set_marker_both (w->start, w->contents, charpos, bytepos);
+    w->window_end_valid = false;
 
-  w->optional_new_start = true;
+    w->optional_new_start = true;
 
-  w->start_at_line_beg = (bytepos == BEGV_BYTE
-                          || FETCH_BYTE (bytepos - 1) == '\n');
+    w->start_at_line_beg = (bytepos == BEGV_BYTE
+                            || FETCH_BYTE (bytepos - 1) == '\n');
 
-  wset_redisplay (w);
+    wset_redisplay (w);
 
-  return Qnil;
+    return Qnil;
+  }
 }
 
 DEFUN ("window-text-width", Fwindow_text_width, Swindow_text_width,
@@ -5836,52 +5897,68 @@ pixels.  */)
 }
 
 DEFUN ("move-to-window-line", Fmove_to_window_line, Smove_to_window_line,
-       1, 1, "P",
+       1, 2, "P\ni",
        doc: /* Position point relative to window.
 ARG nil means position point at center of window.
 Else, ARG specifies vertical position within the window;
-zero means top of window, negative means relative to bottom of window.  */)
-  (Lisp_Object arg)
-{
-  struct window *w = XWINDOW (selected_window);
-  int lines, start;
-  Lisp_Object window;
+zero means top of window, negative means relative to bottom of window.
+
+When GROUP is non-nil, and `move-to-window-line-group-function' is set to a
+function, then instead of the above, that function is called with the
+single argument ARG, and its result returned.
+
+If GROUP is non-nil, and WINDOW is part of a group of windows collectively
+displaying a buffer (such as with Follow Mode), position point relative to
+the group of windows as a whole rather than the individual WINDOW.  This
+condition holds when `move-to-window-line-group-function' is set to a
+function, in which case `move-to-window-line' calls the function with the
+argument ARG, then returns its result, instead of doing its normal
+processing.  */)
+  (Lisp_Object arg, Lisp_Object group)
+{
+  if (!NILP (group)
+      && FUNCTIONP (Vmove_to_window_line_group_function))
+    return call1 (Vmove_to_window_line_group_function, arg);
+  {
+    struct window *w = XWINDOW (selected_window);
+    int lines, start;
+    Lisp_Object window;
 #if false
-  int this_scroll_margin;
+    int this_scroll_margin;
 #endif
 
-  if (!(BUFFERP (w->contents) && XBUFFER (w->contents) == current_buffer))
-    /* This test is needed to make sure PT/PT_BYTE make sense in w->contents
-       when passed below to set_marker_both.  */
-    error ("move-to-window-line called from unrelated buffer");
+    if (!(BUFFERP (w->contents) && XBUFFER (w->contents) == current_buffer))
+      /* This test is needed to make sure PT/PT_BYTE make sense in w->contents
+         when passed below to set_marker_both.  */
+      error ("move-to-window-line called from unrelated buffer");
 
-  window = selected_window;
-  start = marker_position (w->start);
-  if (start < BEGV || start > ZV)
-    {
-      int height = window_internal_height (w);
-      Fvertical_motion (make_number (- (height / 2)), window, Qnil);
-      set_marker_both (w->start, w->contents, PT, PT_BYTE);
-      w->start_at_line_beg = !NILP (Fbolp ());
-      w->force_start = true;
-    }
-  else
-    Fgoto_char (w->start);
+    window = selected_window;
+    start = marker_position (w->start);
+    if (start < BEGV || start > ZV)
+      {
+        int height = window_internal_height (w);
+        Fvertical_motion (make_number (- (height / 2)), window, Qnil);
+        set_marker_both (w->start, w->contents, PT, PT_BYTE);
+        w->start_at_line_beg = !NILP (Fbolp ());
+        w->force_start = true;
+      }
+    else
+      Fgoto_char (w->start);
 
-  lines = displayed_window_lines (w);
+    lines = displayed_window_lines (w);
 
 #if false
-  this_scroll_margin = max (0, min (scroll_margin, lines / 4));
+    this_scroll_margin = max (0, min (scroll_margin, lines / 4));
 #endif
 
-  if (NILP (arg))
-    XSETFASTINT (arg, lines / 2);
-  else
-    {
-      EMACS_INT iarg = XINT (Fprefix_numeric_value (arg));
+    if (NILP (arg))
+      XSETFASTINT (arg, lines / 2);
+    else
+      {
+        EMACS_INT iarg = XINT (Fprefix_numeric_value (arg));
 
-      if (iarg < 0)
-        iarg = iarg + lines;
+        if (iarg < 0)
+          iarg = iarg + lines;
 
 #if false /* This code would prevent move-to-window-line from moving point
              to a place inside the scroll margins (which would cause the
@@ -5889,19 +5966,20 @@ zero means top of window, negative means relative to bottom of window.  */)
              it is probably better not to install it.  However, it is here
              inside #if false so as not to lose it.  -- rms.  */
 
-      /* Don't let it get into the margin at either top or bottom.  */
-      iarg = max (iarg, this_scroll_margin);
-      iarg = min (iarg, lines - this_scroll_margin - 1);
+        /* Don't let it get into the margin at either top or bottom.  */
+        iarg = max (iarg, this_scroll_margin);
+        iarg = min (iarg, lines - this_scroll_margin - 1);
 #endif
 
-      arg = make_number (iarg);
-    }
+        arg = make_number (iarg);
+      }
 
-  /* Skip past a partially visible first line.  */
-  if (w->vscroll)
-    XSETINT (arg, XINT (arg) + 1);
+    /* Skip past a partially visible first line.  */
+    if (w->vscroll)
+      XSETINT (arg, XINT (arg) + 1);
 
-  return Fvertical_motion (arg, window, Qnil);
+    return Fvertical_motion (arg, window, Qnil);
+  }
 }
 
 
@@ -7175,6 +7253,12 @@ syms_of_window (void)
   DEFSYM (Qclone_of, "clone-of");
   DEFSYM (Qfloor, "floor");
   DEFSYM (Qceiling, "ceiling");
+  DEFSYM (Qwindow_start_group_function, "window-start-group-function");
+  DEFSYM (Qwindow_end_group_function, "window-end-group-function");
+  DEFSYM (Qset_window_start_group_function, "set-window-start-group-function");
+  DEFSYM (Qrecenter_group_function, "recenter-group-function");
+  DEFSYM (Qpos_visible_in_window_p_group_function, "pos-visible-in-window-p-group-function");
+  DEFSYM (Qmove_to_window_line_group_function, "move-to-window-line-group-function");
 
   staticpro (&Vwindow_list);
 
@@ -7346,6 +7430,72 @@ Note that this optimization can cause the portion of the buffer
 displayed after a scrolling operation to be somewhat inaccurate.  */);
   Vfast_but_imprecise_scrolling = false;
 
+  DEFVAR_LISP ("window-start-group-function", Vwindow_start_group_function,
+               doc: /* Function to call for `window-start' when its GROUP parameter is non-nil.
+When this variable contains a function, and `window-start' is called with a
+non-nil GROUP parameter, the function is called instead of `window-start''s
+normal action.  `window-start' passes the function its argument WINDOW, which
+might be nil.  The function may recursively call `window-start'.  */);
+  Vwindow_start_group_function = Qnil;
+  Fmake_variable_buffer_local (Qwindow_start_group_function);
+  Fput (Qwindow_start_group_function, Qpermanent_local, Qt);
+
+  DEFVAR_LISP ("window-end-group-function", Vwindow_end_group_function,
+               doc: /* Function to call for `window-end' if its GROUP parameter is non-nil.
+When this variable contains a function, and `window-end' is called with a
+non-nil GROUP parameter, the function is called instead of `window-end''s
+normal action.  `window-end' passes the function its two parameters WINDOW,
+and UPDATE.  The function may call `window-end' recursively.  */);
+  Vwindow_end_group_function = Qnil;
+  Fmake_variable_buffer_local (Qwindow_end_group_function);
+  Fput (Qwindow_end_group_function, Qpermanent_local, Qt);
+
+  DEFVAR_LISP ("set-window-start-group-function",
+               Vset_window_start_group_function,
+               doc: /* The function to call for `set-window-start' when its GROUP parameter is non-nil.
+When this variable contains a function, and `set-window-start' is called
+with a non-nil GROUP parameter, the function is called instead of
+`set-window-start''s normal action.  `set-window-start' passes the function
+its three parameters WINDOW, POS, and NOFORCE.  The function may call
+`set-window-start' recursively.  */);
+  Vset_window_start_group_function = Qnil;
+  Fmake_variable_buffer_local (Qset_window_start_group_function);
+  Fput (Qset_window_start_group_function, Qpermanent_local, Qt);
+
+  DEFVAR_LISP ("recenter-group-function", Vrecenter_group_function,
+               doc: /* Function to call for `recenter' when its GROUP parameter is non-nil.
+When this variable contains a function, and `recenter' is called with a
+non-nil GROUP parameter, the function is called instead of `recenter''s
+normal action.  `recenter' passes the function its parameter ARG.  The
+function may call `recenter' recursively.  */);
+  Vrecenter_group_function = Qnil;
+  Fmake_variable_buffer_local (Qrecenter_group_function);
+  Fput (Qrecenter_group_function, Qpermanent_local, Qt);
+
+  DEFVAR_LISP ("pos-visible-in-window-p-group-function",
+               Vpos_visible_in_window_p_group_function,
+               doc: /*  Function for `pos-visible-in-window-p' when its GROUP parameter is non-nil.
+When this variable contains a function, and `pos-visible-in-window-p' is
+called with a non-nil GROUP parameter, the function is called instead of
+`pos-visible-in-window-p''s normal action.  `pos-visible-in-window-p'
+passes the function its three parameters POS, WINDOW, and PARTIALLY.  The
+function may call `pos-visible-in-window-p' recursively.  */);
+  Vpos_visible_in_window_p_group_function = Qnil;
+  Fmake_variable_buffer_local (Qpos_visible_in_window_p_group_function);
+  Fput (Qpos_visible_in_window_p_group_function, Qpermanent_local, Qt);
+
+  DEFVAR_LISP ("move-to-window-line-group-function",
+               Vmove_to_window_line_group_function,
+               doc: /* Function for `move-to-window-line' when its GROUP parameter is non-nil.
+When this variable contains a function, and `move-to-window-line' is
+called with a non-nil GROUP parameter, the function is called instead of
+`move-to-window-line''s normal action.  `move-to-window-line' passes the
+function its parameter ARG.  The function may call `move-to-window-line'
+recursively.  */);
+  Vmove_to_window_line_group_function = Qnil;
+  Fmake_variable_buffer_local (Qmove_to_window_line_group_function);
+  Fput (Qmove_to_window_line_group_function, Qpermanent_local, Qt);
+
   defsubr (&Sselected_window);
   defsubr (&Sminibuffer_window);
   defsubr (&Swindow_minibuffer_p);