Emacs -- calculating new window-start/end without redisplay

Question

Is it possible to calculate a new window-start/end without a redisplay occurring? If so, then an example would be greatly appreciated. If not, then what is the best way to approximate it? Example: We want to move to a new area of the buffer somewhere off screen, and place overlays when we get there. We might be using page-down or scroll-down or paragraph-down or end-of-buffer. When we get to that new point, we want to calculate the new `window-start` and the new `window-end`. However, we want to avoid a momentary naked looking buffer without any overlays. Ideally, the redisplay would occur once those overlays are added. I want to restrict new overlays to the new region based upon the new window-start/end. Point-Min: point = 1 Old Window Start: point = 1000 Old Window End: point = 1500 New Window Start: point = 3500 New Window End: point = 4000 Point-Max: point = 6000 Problem: When using the `post-command-hook` to try and calculate the new `window-start` and new `window-end`, the previ…

Accepted Answer

See also Emacs bug tracker feature request #22404 (which has not yet been implemented, but the mailing archive contains a rough draft rudimentary patch that creates a new hook for this specific issue): https://debbugs.gnu.org/cgi/bugreport.cgi?bug=22404

- Minor-mode for testing `window-start` and `window-end` BEFORE visual redisplay.

```
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; test-mode
;; A minor-mode for testing `window-start` / `window-end` BEFORE visual redisplay.

(defvar test-this-command nil
"This local variable is set within the `post-command-hook`; and,
is also used by the `window-scroll-functions` hook.")
(make-variable-buffer-local 'test-this-command)

(defun test-post-command-hook-fn ()
"A function attached to the `post-command-hook`."
  (setq test-this-command this-command)
  (test-demo-fn))

(defun test-window-scroll-functions-fn (win _start)
"A function attached to the `window-scroll-functions` hook."
  (test-demo-fn))

(defun test-demo-fn ()
"This is a test-mode demonstration function."
  (when
      (and
        test-mode
        test-this-command
        (window-live-p (get-buffer-window (current-buffer)))
        (not (minibufferp))
        (pos-visible-in-window-p (point)
          (get-buffer-window (current-buffer) (selected-frame)) t))
    (let* (
        (selected-window (selected-window))
        (window-start (window-start selected-window))
        (window-end (window-end selected-window t)) )
      (message "window-start: %s | window-end: %s" window-start window-end)
      (setq test-this-command nil) )))

(define-minor-mode test-mode
"A minor-mode for testing `window-start` / `window-end` BEFORE visual redisplay."
  :init-value nil
  :lighter " TEST"
  :keymap nil
  :global nil
  :group nil
  (cond
    (test-mode
      (set (make-local-variable 'scroll-conservatively) 101)
      (add-hook 'post-command-hook 'test-post-command-hook-fn nil t)
      (add-hook 'window-scroll-functions 'test-window-scroll-functions-fn nil t)
      (when (called-interactively-p 'any)
        (message "Turned ON `test-mode`.")))
    (t
      (kill-local-variable 'scroll-conservatively)
      (kill-local-variable 'test-this-command)
      (remove-hook 'post-command-hook 'test-post-command-hook-fn t)
      (remove-hook 'window-scroll-functions 'test-window-scroll-functions-fn t)
      (when (called-interactively-p 'any)
        (message "Turned OFF `test-mode`.") ))))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
```

Emacs -- calculating new window-start/end without redisplay

Solution

Problem