Marco Wahl <marcowahlsoft@gmail.com> writes:

> Precondition:
>
> - Have a dired with several subdirectories open.

Also, M-x load-lib RET page-ext RET

> Action:
>
> - C-x C-p C-n
>
> Expectation:
>
> - The dired buffer narrows to the next subdirectory.
>
> Observation:
>
> - The dired buffer narrows to the current subdirectory.

> * lisp/textmodes/page-ext.el (next-page): Jump ahead page-delimiter if
>   at such before narrow.  This fixes the command for dired.

I think the problem is rather that next-page is going an extra page
backwards even when COUNT was positive, so the fix should be more like
this (inline version with whitespace changes ignored, full version
attached):

--- a/lisp/textmodes/page-ext.el
+++ b/lisp/textmodes/page-ext.el
@@ -304,6 +304,7 @@ next-page
   (or count (setq count 1))
   (widen)
   ;; Cannot use forward-page because of problems at page boundaries.
+  (if (>= count 0)
       (while (and (> count 0) (not (eobp)))
         (if (re-search-forward page-delimiter nil t)
             nil
@@ -316,7 +317,7 @@ next-page
       (if (re-search-backward page-delimiter nil t)
           (goto-char (match-beginning 0))
         (goto-char (point-min)))
-    (setq count (1+ count)))
+      (setq count (1+ count))))
   (narrow-to-page)
   (goto-char (point-min))
   (recenter 0))