bug#4041: 23.0.92; Emacs 23: buffer point is no longer frame-local

[martin rudalics <rudalics@gmx.at>]

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 > That looks pretty good.  I'd be willing to try and turn it on by default
 > in something like Emacs-25 or maybe even 24.2.

I completely forgot about this.  Any objections to install the attached
version for 24.3?

 > One thing bothers me, tho: why not do it in set-window-buffer?

I'm still not sure whether this would be safe in all cases.  I wouldn't
even do that for plain `display-buffer'.  The current patch addresses
all requests raised in the current thread.  If there's real need to
provide this option for other functions as well, we can still do that.

martin

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=== modified file 'lisp/ChangeLog'
--- lisp/ChangeLog	2012-10-04 12:56:14 +0000
+++ lisp/ChangeLog	2012-10-05 09:40:16 +0000
@@ -1,3 +1,10 @@
+2012-10-05  Martin Rudalics  <rudalics@gmx.at>
+
+	* window.el (switch-to-buffer-preserve-window-point): New
+	option.
+	(switch-to-buffer): Optionally preserve window point if buffer
+	appeared earlier in the selected window (Bug#4041).
+
 2012-10-04  Martin Rudalics  <rudalics@gmx.at>
 
 	* emacs-lisp/edebug.el (edebug-pop-to-buffer): Select window

=== modified file 'lisp/window.el'
--- lisp/window.el	2012-09-30 10:44:43 +0000
+++ lisp/window.el	2012-10-05 09:47:10 +0000
@@ -5812,6 +5812,25 @@
 	    buffer))
     (other-buffer)))
 
+(defcustom switch-to-buffer-preserve-window-point nil
+  "If non-nil, `switch-to-buffer' tries to preserve `window-point'.
+If this is nil, `switch-to-buffer' displays the buffer at that
+buffer's `point'.  If this is `already-displayed', it tries to
+display the buffer at its last position in the selected window
+provided the buffer is curently displayed in some other window on
+a visible or iconified frame.  If this is t, it always tries to
+display the buffer at its last position in the selected window.
+
+If the window used for display is either new, or the buffer
+already appears in it, or the buffer never appeared in that
+window, the setting of this variable has no impact."
+  :type '(choice
+	  (const :tag "Never" nil)
+	  (const :tag "If already displayed elsewhere" already-displayed)
+	  (const :tag "Always" t))
+  :group 'windows
+  :version "24.3")
+
 (defun switch-to-buffer (buffer-or-name &optional norecord force-same-window)
   "Switch to buffer BUFFER-OR-NAME in the selected window.
 If the selected window cannot display the specified
@@ -5837,6 +5856,10 @@
 must be displayed in the selected window; if that is impossible,
 signal an error rather than calling `pop-to-buffer'.
 
+The option `switch-to-buffer-preserve-window-point' can be used
+to make the buffer appear at its last position in the selected
+window.
+
 Return the buffer switched to."
   (interactive
    (list (read-buffer-to-switch "Switch to buffer: ") nil 'force-same-window))
@@ -5853,7 +5876,21 @@
       (if force-same-window
           (user-error "Cannot switch buffers in a dedicated window")
         (pop-to-buffer buffer norecord)))
-     (t (set-window-buffer nil buffer)))
+     (t
+      (let* ((entry (assq buffer (window-prev-buffers)))
+	     (displayed (and (eq switch-to-buffer-preserve-window-point
+				 'already-displayed)
+			     (get-buffer-window buffer 0)))
+	     (start (and entry (nth 1 entry)))
+	     (pos (and entry (nth 2 entry))))
+	(set-window-buffer nil buffer)
+	(when (and entry
+		   (or (eq switch-to-buffer-preserve-window-point t)
+		       displayed))
+	  ;; Try to restore start and point of buffer in the selected
+	  ;; window (Bug#4041).
+	  (set-window-start (selected-window) start t)
+	  (set-window-point nil pos)))))
 
     (unless norecord
       (select-window (selected-window)))