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))

(defun my-scroll-and-clamp-up-command ()
  (interactive)
  (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))

(defun my-scroll-and-clamp-down-command ()
  (interactive)
  (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)