ediff-fullscreen-as-wide-display.el

(defun toggle-frame-fullscreen (&optional frame)
  (interactive)
  (set-frame-parameter frame 'fullscreen
                       (if (null (frame-parameter frame 'fullscreen))
                           'fullboth nil)))

(global-set-key "\C-x59" 'toggle-frame-fullscreen)

; ediff-toggle-wide-display tries to use the entire display width,
; which breaks with multiple monitors
(defun ediff-fullscreen-as-wide-display ()
  (if (eq window-system 'mac)
      (defadvice ediff-toggle-wide-display
        (around ediff-toggle-frame-fullscreen activate)
        (toggle-frame-fullscreen (window-frame ediff-window-A))
        (raise-frame ediff-control-frame))))
(add-hook 'ediff-mode-hook 'ediff-fullscreen-as-wide-display)