When the full-screen window are two ways to start Emacs

# emacs ~/.emacs
;;启动emacs时窗口最大化
;;方式一
(defun fullscreen ()
  (interactive)
  (set-frame-parameter nil 'fullscreen
       (if (frame-parameter nil 'fullscreen) nil 'fullboth)))

(global-set-key [f12] 'fullscreen)

;;方式二
(add-to-list 'default-frame-alist '(fullscreen . maximized))