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 16:32:22 -0400
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> 
>> > 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.
>
> But in that case I don't have a clear idea how to incorporate what you
> want in the way scrolling is implemented in Emacs.  The scrolling
> functions usually don't move point, they just set the window-start
> position according to the scroll-command argument and relevant user
> options.  The actual scrolling is done by the display engine during
> the next redisplay cycle, and that either leaves point where it was
> (if point is still visible)

The code I'd want to modify is...

> or moves it into the viewport.

exactly this code.  e.g. in window_scroll_line_based there's calls to
Fvertical_motion, I'd want to add more logic just before those calls.
I'd need to do the same for window_scroll_pixel_based.

> This scrolling is general display feature, it can happen even if
> something other than a scrolling command caused it.  Thus, knowing
> when to move point to a particular place is not trivial, and would
> need some variable exposed to Lisp or something like that, and then
> some non-trivial logic to use that.

Hm, not sure why I'd need logic in Lisp, if I take the approach I just
mentioned.  (We'd need a defcustom of course to control the behavior,
but other than that.)

> It is also not clear whether you want this only for scroll commands or
> also for other causes of scrolling the window.
>
> Maybe you could do something similar to how
> scroll-preserve-screen-position works.
>
> Feel free to work on this, of course.