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

[martin rudalics]

> I think I generally agree it would be a good change.  But without an
> actual patch, it's difficult to assess how reliable and backward
> compatible we can make it.

Attached find a diff with a new option
`switch-to-buffer-preserve-window-point'.

martin

=== modified file 'lisp/window.el'
--- lisp/window.el      2011-10-10 17:52:03 +0000
+++ lisp/window.el      2011-10-12 09:23:47 +0000
@@ -4976,6 +4976,19 @@
            buffer))
     (other-buffer)))
 
+(defcustom switch-to-buffer-preserve-window-point nil
+  "If non-nil, `switch-to-buffer' tries to preserve `window-point'.
+If nil, `switch-to-buffer' displays the buffer at that buffer's
+`point'.  If non-nil, it tries to display the buffer at the last
+position of `window-point' in the window used for display.
+
+If the window used for display is either new, or the buffer
+already appears in it, or the buffer never appeared in that
+window, this variable has no impact."
+  :type 'boolean
+  :group 'windows
+  :version "24.1")
+
 (defun switch-to-buffer (buffer-or-name &optional norecord force-same-window)
   "Switch to buffer BUFFER-OR-NAME in the selected window.
 If called interactively, prompt for the buffer name using the
@@ -5001,7 +5014,7 @@
 
 Return the buffer switched to."
   (interactive
-   (list (read-buffer-to-switch "Switch to buffer: ") nil nil))
+   (list (read-buffer-to-switch "Switch to buffer: ") nil t))
   (let ((buffer (window-normalize-buffer-to-switch-to buffer-or-name)))
     (if (null force-same-window)
        (pop-to-buffer buffer display-buffer--same-window-action norecord)
@@ -5013,7 +5026,19 @@
        (error "Cannot switch buffers in minibuffer window"))
        ((eq (window-dedicated-p) t)
        (error "Cannot switch buffers in a dedicated window"))
-       (t (set-window-buffer nil buffer)))
+       (t
+       (let* ((entry (and switch-to-buffer-preserve-window-point
+                          (assq buffer (window-prev-buffers))))
+              (start (and entry (nth 1 entry)))
+              (pos (and entry (nth 2 entry))))
+         (set-window-buffer nil buffer)
+         (when entry
+           ;; If BUFFER-OR-NAME (1) was shown in the selected window
+           ;; before and (2) is currently displayed in some other
+           ;; visible window, try to restore start and point of buffer
+           ;; in the selected window.
+           (set-window-start (selected-window) start t)
+           (set-window-point-1 nil pos)))))
 
       (unless norecord
        (select-window (selected-window)))