bug#8789: 23.3; debug backtrace buffer changes window on step-through

[martin rudalics]

> Sounds good for the backtrace buffer, but the patch you propose would
> affect many more cases, and while I think it might be good to do it
> globally like you suggest, I wouldn't want to make such a change so late
> in the pretest.

Neither do I.  The problem is that if a window is not very suitable for
getting reused, there's no easy way for the user (or the application) to
communicate that to the buffer display routines.

> Can you provide another patch that limits the effect to the
> backtrace buffer?

Can you?  I think that using `display-buffer-overriding-action' would be
too harsh so we would have to specify this via the ACTION argument in
(pop-to-buffer debugger-buffer).  A patch in that direction is below.

martin


=== modified file 'lisp/emacs-lisp/debug.el'
--- lisp/emacs-lisp/debug.el    2012-01-19 07:21:25 +0000
+++ lisp/emacs-lisp/debug.el    2012-02-16 07:51:35 +0000
@@ -194,7 +194,9 @@
                  ;; Place an extra debug-on-exit for macro's.
                  (when (eq 'lambda (car-safe (cadr (backtrace-frame 4))))
                    (backtrace-debug 5 t)))
-                (pop-to-buffer debugger-buffer)
+                (pop-to-buffer
+                debugger-buffer
+                '((display-buffer-in-window-previously-showing-it . nil)))
                (debugger-mode)
                (debugger-setup-buffer debugger-args)
                (when noninteractive

=== modified file 'lisp/window.el'
--- lisp/window.el      2012-02-12 05:10:30 +0000
+++ lisp/window.el      2012-02-16 07:54:35 +0000
@@ -4846,6 +4846,22 @@
       (and pop-up-windows
           (display-buffer-pop-up-window buffer alist))))

+(defun display-buffer-in-window-previously-showing-it (buffer &optional alist)
+  "Display BUFFER in a window previously showing it."
+  (let (best-window second-best-window window)
+    (catch 'best-window
+      (dolist (window (window-list-1 nil 'nomini 'visible))
+       (when (and (assq buffer (window-prev-buffers window))
+                  (not (window-dedicated-p window)))
+         (if (eq window (selected-window))
+             (setq second-best-window window)
+           (setq best-window window)
+           (throw 'best-window t)))))
+
+    (when (setq window (or best-window second-best-window))
+      (display-buffer-record-window 'reuse window buffer)
+      (window--display-buffer-2 buffer window display-buffer-mark-dedicated))))
+
 (defun display-buffer-use-some-window (buffer alist)
   "Display BUFFER in an existing window.
 Search for a usable window, set that window to the buffer, and