bug#34723: 27.0.50; customize and improve diff-mode recentering

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> There are cases where recentering does "make sure that the whole entry
>> is visible", but as we saw in the first post of this report, there is
>> at least one case where recentering makes less useful context visible.
> That's clearly undesirable, indeed.  I think it's a plain bug.
> Basically when moving in one direction, diff-hunk-next/prev should never
> scroll the buffer in the other direction.

I installed the patch below which should fix this most glaring problem.


        Stefan


diff --git a/lisp/emacs-lisp/easy-mmode.el b/lisp/emacs-lisp/easy-mmode.el
index b866a95443..57cf5c86f4 100644
--- a/lisp/emacs-lisp/easy-mmode.el
+++ b/lisp/emacs-lisp/easy-mmode.el
@@ -625,6 +625,7 @@ easy-mmode-define-navigation
                  ,body
                  (when was-narrowed (funcall #',narrowfun)))))))
     (unless name (setq name base-name))
+    ;; FIXME: Move most of those functions's bodies to helper functions!
     `(progn
        (defun ,next-sym (&optional count)
 	 ,(format "Go to the next COUNT'th %s.
@@ -646,7 +647,11 @@ easy-mmode-define-navigation
                                         `(re-search-forward ,re nil t 2)))
                                    (point-max))))
                     (unless (pos-visible-in-window-p endpt nil t)
-                      (recenter '(0)))))))
+                      (let ((ws (window-start)))
+                        (recenter '(0))
+                        (if (< (window-start) ws)
+                            ;; recenter scrolled in the wrong direction!
+                            (set-window-start nil ws))))))))
            ,@body))
        (put ',next-sym 'definition-name ',base)
        (defun ,prev-sym (&optional count)