;; generic code:
(defvar predisplay-hook nil
  "Normal hook run before predisplay.")
(defvar window-changed-size-hook nil
  "Normal hook run when a window displaying this buffer changed size.")
(defvar predisplay-sizes (make-hash-table :weakness 'key :test 'eq)
  "Hash table of old window sizes to detect size changes.")

(defun predisplay-check-window ()
  "Run in hook `predisplay-hook' to determine which windows changed size."
  (let* ((w (selected-window))
	 (old-size (gethash w predisplay-sizes))
	 (new-size (cons (window-pixel-width w)
			 (window-pixel-height w))))
    (unless (equal old-size new-size)
      (run-hooks 'window-changed-size-hook)
      (puthash w new-size predisplay-sizes))))

(add-hook 'predisplay-hook #'predisplay-check-window)

(defun predisplay-function (&rest args)
  (dolist (w (window-list-1 nil t t))
    (with-selected-window w
      (run-hooks 'predisplay-hook))))

(add-function :before pre-redisplay-function
              #'predisplay-function)
;; demonstration code:
(defun my-window-changed-size ()
  (insert (format "%dx%d\n" (window-pixel-width (selected-window))
                  (window-pixel-height (selected-window)))))

(defun display-dynamic-size ()
  (add-hook 'window-changed-size-hook #'my-window-changed-size nil t))

;; dead code:

;; I thought this would work, but it doesn't, for reasons I believe
;; Eli described.

;; (defun predisplay-run-hook (window)
;;   (with-selected-window window
;;     (run-hooks 'predisplay-hook)))

;; (push #'predisplay-run-hook pre-redisplay-functions)