Can page-down clamp the last line to the bottom of the window?

Question

When using page-down (internally scroll-up-command), sometimes the window ends up in a state where end end-of-file is at the top of the screen.

Is there a way to page-down, clamping window position so it doesn't end up scrolling past the document end?

The emacs buffer displays something like this:

;;; some-file.el ends here

---

Note that I'm not asking for a way to prevent the window moving past the document end entirely (although that could be nice), just a way to change the behavior of scroll-up-command

Answer 1

This can be done using scrolling commands written in elisp.

;; Scrolling that re-centers, keeping the cursor vertically centered.
;; Also clamps window top when scrolling down,
;; so the text doesn't scroll off-screen.

(defun my-scroll-and-clamp--forward-line (n)
  "Wrap `forward-line', supporting Emacs built-in goal column.
Argument N the number of lines, passed to `forward-line'."
  (let ((next-column
         (or goal-column
             (and (memq last-command
                        '(next-line previous-line line-move))
                  (if (consp temporary-goal-column)
                      (car temporary-goal-column)
                    temporary-goal-column)))))
    (unless next-column
      (setq temporary-goal-column (current-column))
      (setq next-column temporary-goal-column))
    (forward-line n)
    (move-to-column next-column))
  ;; Needed so `temporary-goal-column' is respected in the future.
  (setq this-command 'line-move))

(defmacro my-scroll-and-clamp--with-evil-visual-mode-hack (&rest body)
  "Execute BODY with the point not restricted to line limits.

This is needed so the point is not forced to line bounds
even when in evil visual line mode."
  `(let ((mark-found nil))
     (when (and (fboundp 'evil-visual-state-p)
                (funcall 'evil-visual-state-p)
                (fboundp 'evil-visual-type)
                (eq (funcall 'evil-visual-type) 'line)
                (boundp 'evil-visual-point))
       (let ((mark (symbol-value 'evil-visual-point)))
         (when (markerp mark)
           (setq mark-found mark))))
     (unwind-protect
         (progn
           (when mark-found
             (goto-char (marker-position mark-found)))
           ,@body)
       (when mark-found
         (set-marker mark-found (point))))))


;;;###autoload
(defun my-scroll-and-clamp-up-command ()
  (interactive)
  (my-scroll-and-clamp--with-evil-visual-mode-hack
   (let ((height (window-height)))

     ;; Move point.
     (my-scroll-and-clamp--forward-line height)

     ;; Move window.
     (set-window-start
      (selected-window)
      (min
       (save-excursion ;; new point.
         (forward-line (- (/ height 2)))
         (point))
       (save-excursion ;; max point.
         (goto-char (point-max))
         (beginning-of-line)
         (forward-line (- (- height (+ 1 (* 2 scroll-margin)))))
         (point))))))
  (redisplay))

;;;###autoload
(defun my-scroll-and-clamp-down-command ()
  (interactive)
  (my-scroll-and-clamp--with-evil-visual-mode-hack
   (let* ((height (window-height)))

     ;; Move point.
     (my-scroll-and-clamp--forward-line (- height))
     (setq this-command 'line-move)

     ;; Move window.
     (set-window-start
      (selected-window)
      (save-excursion ;; new point.
        (forward-line (- (/ height 2)))
        (point)))))
  (redisplay))

Example shortcut bindings:

(global-set-key (kbd "<next>") 'my-scroll-and-clamp-up-command)
(global-set-key (kbd "<prior>") 'my-scroll-and-clamp-down-command)