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I've written a function that recenters the point each time it makes a long jump and I added it to the hook window-scroll-functions. The length of the jump is determined by comparing the return value of (line-number-at-pos (window-end)) to that of (line-number-at-pos (window-end nil 'update)), where the former holds the line number at the window-end position before the jump, and the latter after the jump.

The problem is that window-scroll-functions runs also when I change the buffer displayed by the current window. In that case (line-number-at-pos (window-end)) returns the line number at the window-end position of the previous buffer, and I can't figure out which one it was because (ref.):

At the time of the call, the display-start position of the argument window is already set to its new value, and the buffer to be displayed in the window is set as the current buffer.

If the window-end of the previous buffer and the new one differ enough, my recentering function gets called even though it shouldn't.

How can I find out whether it was a buffer change that triggered window-scroll-functions?

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  • 30 years ago, I did something similar in follow-mode. My solution was to use pre-command-hook and store information before a function and check it in post-command-hook. If they matched I would scroll the windows, and windows close by, to create the illusion of one very tall window.
    – Lindydancer
    Mar 21, 2022 at 15:35
  • A totally different approach would be to define a number of functions that make "long jumps", and bind them to suitable keys. Of course, this approach would only work if you can identify the functions you are interested in handling.
    – Lindydancer
    Mar 21, 2022 at 15:37

1 Answer 1

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One way to do it is by comparing the current-time with the buffer-display-time. If the gap is small enough it means it was most likely a buffer change that ran the hook.

(defun my-test-scroll-or-buffer-change (window _window-start-after)
  (with-selected-window window
    (if (> 0.001 (float-time (time-subtract (current-time) buffer-display-time)))
        (message "It was a buffer change")
      (message "it was a scroll"))))

(add-hook 'window-scroll-functions #'my-test-scroll-or-buffer-change)

Not very pretty but it seems to work.

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