bug#32863: Unsatisfactory "definition" of "vertical scroll position" in Emacs lisp manual and doc string of window-vscroll

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 15 Oct 2018 11:48:03 +0000
> Cc: 32863@debbugs.gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> I propose to amend windows.texi, and two doc strings in windows.c in the
> emacs-26 branch as follows.  I think that these amendments would have
> prevented my initial puzzlement.  Any comments?

The changes to doc strings are fine (but please use "display" instead
of "redisplay", as the latter is a technical term not necessarily
accurate in this context).

As for the changes to windows.texi, I admit that I don't really
understand how such insignificant changes could prevent your
puzzlement.  Are you sure it isn't the result of reading my
explanations, which caused the text to "talk" to you more clearly?

>     @dfn{Vertical fractional scrolling} means shifting text in a window
> -up or down by a specified multiple or fraction of a line.  Each window
> +up or down by a specified multiple or fraction of a line.  Emacs uses it
> +on images and text rows which are taller than the window.  Each window

Here you added a sentence that tells when is this feature used.  (Btw,
nothing prevents Emacs from using it even with lines that are not
taller than the window, and it even does so sometimes.  So this is
just an example, and I'd suggest to say that.  Also, please don't use
"rows" in the manual: that is the terminology of xdisp.c, but Lisp
programmers are unfamiliar with it.  I prefer to use "screen lines".)

>  has a @dfn{vertical scroll position}, which is a number, never less than
> -zero.  It specifies how far to raise the contents of the window.
> -Raising the window contents generally makes all or part of some lines
> -disappear off the top, and all or part of some other lines appear at the
> -bottom.  The usual value is zero.
> +zero.  It specifies how far to raise the contents of the window when
> +redisplaying it.  Raising the window contents generally makes all or
> +part of some lines disappear off the top, and all or part of some other
> +lines appear at the bottom.  The usual value is zero.

Here you just added "when redisplaying it".  If this is such a
crucially important detail, I'm all for it (but again, please say
"displaying").  However, I wonder whether it really is all that
stood between you and the eureka.

>     The vertical scroll position is measured in units of the normal line
>  height, which is the height of the default font.  Thus, if the value is
> -.5, that means the window contents are scrolled up half the normal line
> -height.  If it is 3.3, that means the window contents are scrolled up
> -somewhat over three times the normal line height.
> +.5, that means the window contents will be scrolled up half the normal
> +line height.  If it is 3.3, that means the window contents are scrolled
> +up somewhat over three times the normal line height.

And here, you just replaced "are scrolled" with "will be scrolled"
(only once out of 2 instances).  Again, one wonders whether this is
all that you missed to see the light.

I'm not saying I object to these changes -- I don't -- but I'm a bit
surprised that such minor changes could have such a profound effect on
clarity.

Thanks.