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bug#32863: Unsatisfactory "definition" of "vertical scroll position" in
bug#32863: Unsatisfactory "definition" of "vertical scroll position" in Emacs lisp manual and doc string of window-vscroll
Sat, 29 Sep 2018 00:41:54 +0300
> Date: Fri, 28 Sep 2018 15:28:32 +0000
> From: Alan Mackenzie <address@hidden>
> In the Emacs-26.1 Emacs Lisp manual, on page "Vertical Scrolling" there
> is an ostensible definition for "vertical scroll position".
> This "definition" says it "is a number, never less than zero. It
> specifies how far to raise the contents of the window."
> What should be doing this raising?
The call to set-window-vscroll does that.
> When might it do this?
The automatic calls to set-window-vscroll happen when a screen line is
too tall to be completely visible in a window, and Emacs is asked to
scroll. Try scrolling or vertical motion commands when the window
displays a tall image.
> For what purpose might the contents of the window be raised?
To show the portion of the current line that is not currently visible.
> I find this "definition" totally obscure. I can not make sense of it at
> all. Without understanding what "vertical scroll position" means, the
> entire manual page is meaningless.
Do you understand it now?
> I came to this manual page through not understanding the doc string for
> the function window-vscroll. This says just "Return the amount by which
> WINDOW is scrolled vertically.".
> _Is_ scrolled vertically. What on earth does that mean?
> What is the zero point from which this scrolling is measured?
The zero point is when the top pixel of the window's first screen line
> Does this "is" refer to the current visible scrolling, or the
> intended scrolling after the next redisplay?
Since redisplay runs when Emacs is idle, the answer should be obvious,
> This doc string needs clarification.
I'm all ears.