Maximized undecorated (title-less) Emacs frame with no gaps around

toggle-frame-maximized-undecorated.el

(defun toggle-frame-maximized-undecorated ()
  (interactive)
  (let* ((frame (selected-frame))
         (on? (and (frame-parameter frame 'undecorated)
                   (eq (frame-parameter frame 'fullscreen) 'maximized)))
         (geom (frame-monitor-attribute 'geometry))
         (x (first geom))
         (y (second geom))
         (display-height (first (last geom))))
    (if on?
        (progn
          (set-frame-parameter frame 'undecorated nil)
          (toggle-frame-maximized))
      (progn
        (set-frame-position frame x y)
        (set-frame-parameter frame 'fullscreen 'maximized)
        (set-frame-parameter frame 'undecorated t)
        (set-frame-height frame (- display-height 26) nil t)
        (set-frame-position frame x y)))))