wrap-column.el

(defun wc/narrow-window ()
  (let (( new-right
          (max 0 (+ (or (cdr (window-margins)) 0)
                    (- (window-body-width) fill-column)))))
    (set-window-margins nil (car (window-margins)) new-right)
    (set-window-fringes nil (car (window-fringes)) 2)))

(define-minor-mode wrap-column-mode
    "Wrap the text at `fill-column'.
Works by adjusting the right margin."
  nil nil nil
  (if wrap-column-mode
      (progn
        (visual-line-mode 1)
        (add-hook 'window-configuration-change-hook
                  'wc/narrow-window nil t)
        (wc/narrow-window))
      (progn
        (remove-hook 'window-configuration-change-hook
                     'wc/narrow-window t)
        (set-window-margins nil (car (window-margins)) nil)
        (set-window-fringes nil (car (window-fringes))
                                (car (window-fringes))))))

(provide 'wrap-column)