Use dirtree-expand instead of `find-file-other-window- when :file is a directory

[vermiculus]

I'd like to stay in dirtree when click/RET/whatever on a directory name (and not just the icon).

I've tried patching dirtree-select with very limited success:

(defun dirtree-select (node &rest ignore)
  "Open file in other window"
  (let ((file (widget-get node :file)))
    (and file
         (if (file-directory-p file) ;note
             (dirtree-expand
              (progn
                (beginning-of-line)
                (widget-forward 1)
                (widget-at (point))))
           (find-file-other-window file)))))

but I cannot seem to get dirtree-expand to cooperate. Can there be a better way to do this? I think C-o is a completely reasonable way to get out of dirtree into dired if so desired – I don't think there'll be a loss of functionality.