With this patch, "RET" works as expected, while "<mouse-1>" is giving an
error for a reason I don't understand.  In order to make it work for the
mouse, I have to do something contorted instead of a plain
`make-button':

(define-key (overlay-get
             (make-button opoint (point)
                          'follow-link t
                          'action (lambda (&rest _x) (find-face-definition symbol)))
             'keymap) [down-mouse-1]
  (lambda ()
    (interactive)
    (let ((button (button-at (point))))
      (when button
        (button-activate button)
        t))))

Maybe someone can suggest a better way.

Oleh