bug#70386: 30.0.50; (recenter 0 t) does not put point on top of the window

[Eli Zaretskii <eliz@gnu.org>]

Ping!  Po Lu, could you please look into this issue?

> Cc: 70386@debbugs.gnu.org
> Date: Mon, 15 Apr 2024 19:11:31 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > From: Ihor Radchenko <yantar92@posteo.net>
> > Cc: 70386@debbugs.gnu.org
> > Date: Sun, 14 Apr 2024 19:36:04 +0000
> > 
> > Eli Zaretskii <eliz@gnu.org> writes:
> > 
> > >> 1. emacs -Q
> > >> 2. Insert the following into scratch buffer and put point at "recenter"
> > >
> > > Which "recenter": the one in the comment or the one in (recenter 0 t) ?
> > 
> > The "(recenter 0 t))" line.
> > 
> > >> (progn
> > >>   (require 'pixel-scroll)
> > >>   (pixel-scroll-precision-interpolate
> > >>    (* -1 (line-pixel-height)
> > >>       (max 0 (- (count-screen-lines (window-start) (point)) 2)))
> > >>    nil 1)
> > >>   ;; Call original recenter for final adjustment.
> > >>   (recenter 0 t))
> > >> 
> > >> 3. M-x eval-buffer
> > >> 4. Observe that scroll is not set to line 0, despite calling `recenter'
> > >
> > > I don't think I understand what you mean by "scroll is not set to line
> > > 0".  Please explain in more detail what you expected to happen (and
> > > why), and what did happen.
> > 
> > Expected is as per docstring:
> > 
> >     With a numeric prefix argument ARG, recenter putting point on screen line ARG
> >     relative to the selected window.
> > 
> > So, I expect that (recenter 0 t) will put the line at point on top of
> > the window and that (recenter 1 t) will put the line at point at the
> > second top line.
> > 
> > My expectation is met when I simply do emacs -Q M-: (recenter 0 t) or
> > emacs -Q M-: (recenter 1 t) on master.
> > 
> > Observed:
> > 
> > 1. Line at point slowly scrolls up until it reaches top of the window
> > (`pixel-scroll-precision-interpolate' call)
> > 
> > 2. Line at point is reset back to its initial scroll position
> >    (`recenter' call)
> 
> What is the "initial scroll position" in this case?
> 
> > Sometimes, I also observe line at point moving beyond the screen.
> 
> It's inconsistent in my testing: sometimes works as I'd expect, and
> sometimes ends up with window's vscroll that is very small: about 1
> screen line.
> 
> Po Lu, could you please look into this?  Something in
> pixel-scroll-precision-interpolate is randomly misbehaving, at least
> on my system.  Since all of this stuff is extremely fragile, it's
> possible that one of the recent changes in xdisp.c broke it somehow.
> But the behavior is not consistent, so I wonder what could cause that.
> 
> 
> 
>