Sorry, the right fringe renders correctly, but the margin still overflows. Attached is a screenshot of magit-log, where this can be seen. Below is a test case.

Evgeni

(let ((ov (make-overlay (point-min) (point-max))))
  (overlay-put ov
               'before-string
               (propertize "o" 'display
                           (list '(margin right-margin)
                                 (propertize
                                  "Laetamur quis quam, aliqua nostrum."
                                  'face '(:background "yellow")
                                  ))))
  (set-window-margins nil (car (window-margins)) 30))