Re: Make scroll-{up, down} move point to {start, end} of newly visible text

[Spencer Baugh <sbaugh@janestreet.com>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Spencer Baugh <sbaugh@janestreet.com>
>> Date: Wed, 16 Aug 2023 13:55:42 -0400
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> 
>> >> (defun my-scroll-up-command (&optional arg)
>> >>   (interactive "^P")
>> >>   (move-to-window-line -1)
>> >>   (scroll-up-command arg))
>> >> 
>> >> (defun my-scroll-down-command (&optional arg)
>> >>   (interactive "^P")
>> >>   (move-to-window-line 0)
>> >>   (scroll-down-command arg))
>> >> 
>> >> except of course this implementation moves point even if it's not
>> >> necessary.
>> >> 
>> >> Does this seem like a reasonable feature to add to the scrolling code?
>> >
>> > I don't understand what you have in mind.  The above commands seem to
>> > do what you want only when ARG is 1.  Or what did I miss?
>> 
>> They're actually wrong when ARG is 1, because they move point without
>> needing to.  They mostly only work when ARG is nil.
>
> That's what I meant: only 1 widow-full scroll.
>
>> See the prose description of what I want, the commands are close to but
>> not exactly right because they move point unnecessarily:
>> 
>> >> When point is off-screen after scroll-up, we should first try to move
>> >> point to "window-end before we scrolled", and only if that position is
>> >> also off-screen should we move point to window-start.  (Currently we
>> >> unconditionally move point to window-start)
>> 
>> > For scrolling just one window-full, setting next-screen-context-lines
>> > to 1 should do what you want.  For scrolling more than that, I don't
>> > understand how to generalize what you want.
>> 
>> The prose description I gave generalizes just fine, doesn't it?  The
>> prose description is completely invariant to how much scrolling is done.
>
> Either setting next-screen-context-lines to 1 should do what you want,
> or I still don't understand what you want when scrolling by more than
> one window-full.

Well, yes, if next-screen-context-lines is 1 then
window-end-before-scroll and window-start-after-scroll are the same.  So
that trivially provides the behavior I described, I suppose...

But I want the behavior I described even when next-screen-context-lines
is greater than 1.  (I personally set it to 10.)  In that case,
window-end-before-scroll and window-start-after-scroll aren't the same,
and Emacs doesn't behave how I described.