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Q: is there a way to set different scroll margins at the top and bottom of a window?

The variable scroll-margin (defined in the C source code) sets how many lines of text that Emacs displays, at a minimum, at the top and bottom of a window:

Number of lines of margin at the top and bottom of a window. Recenter the window whenever point gets within this many lines of the top or bottom of the window.

I'd like to use scroll-margin for the bottom of the window, but don't want it at the top (e.g., a positive value at the bottom, but 0 at the top). Is there a way to do so?

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  • I don't know the answer, but in all likelihood scroll-margin is not the right tool for the job. You may need some function to put on post-command-hook to determine whether to scroll the window. – Harald Hanche-Olsen Jan 19 '15 at 15:29
  • To follow up on my previous comment, finding out how many lines are visible around point seems highly nontrivial, considering wrapped lines, hidden text and the like. I suspect the necessary information is easily available only inside the display code. You could try submitting a feature request to allow (for example) a cons (top . bottom) for scroll-margin. – Harald Hanche-Olsen Jan 19 '15 at 17:25
  • You might be interested in playing with the window-scroll-functions hook (which takes two arguments) and putting a recenter command into a function attached to that hook and then use a setting of (setq scroll-conservatively 101) -- in essence, you define when recentering occurs and turn off the built-in auto-scroll. Something like (when (and (>= (point) (window-end nil t)) (not (pos-visible-in-window-p (point) (get-buffer-window (current-buffer) (selected-frame))))) (recenter)) – lawlist Jan 19 '15 at 18:41
  • @lawlist: thanks for the suggestion, but the end of the docstring reads "Warning: Do not use this feature to alter the way the window is scrolled. It is not designed for that, and such use probably won't work." – Dan♦ Jan 19 '15 at 19:17
  • Interesting question, I am wondering why there is no comments/vote on the answer provided by PythonNut. – Name Jul 19 '15 at 7:36
1

The following works for me: 

(defun my-define-scroll-margins (&rest _args)
  (interactive)
  (with-demoted-errors
    (let ((computed-margin
            (if (> (cdr (nth 6 (posn-at-point)))
                  (/ (window-body-height) 2)) 5 0)))
      (setq
        scroll-margin computed-margin
        smooth-scroll-margin computed-margin))))

(add-hook 'post-command-hook #'my-define-scroll-margins)

Note that this is theoretically inefficient, but performance on my lame computer seems unaffected.

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