Re: Allowing point to be outside the window?

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: emacs-devel@gnu.org
> Date: Wed, 08 Dec 2021 09:17:13 +0800
> 
>       /* We used to issue a CHECK_MARGINS argument to try_window here,
>        but this causes scrolling to fail when point begins inside
>        the scroll margin (bug#148) -- cyd  */
>       clear_glyph_matrix (w->desired_matrix);
>       if (!try_window (window, startp, 0))
>       {
>         w->force_start = true;
>         clear_glyph_matrix (w->desired_matrix);
>         goto need_larger_matrices;
>       }
> 
> Why does it have to force start?

Because the place for the window-start was given to us (this code is
under the 'if (w->force_start)' condition), but we've reset the flag
after checking the condition, so now we are putting the flag back, for
the next redisplay cycle -- since we are abandoning this cycle.

> Doesn't need_larger_matrices take care of resizing the matrix when,
> for instance, fonts change?

need_larger_matrices indeed resizes the glyph matrices, after aborting
the redisplay cycle with the old matrices, but that doesn't mean that
window-start is invalid, it just means the previous glyph matrices
were not large enough, e.g., because some part of the displayed text
now has a different face that uses a smaller font.

Basically, the strategy of redisplay_window, after some initial
bookkeeping, is to go through a series of steps, whereby if a step
succeeds, we are done, otherwise we go to the next step.  The steps
are:

  . if nothing's changed except point, and point is still inside the
    same window, no need to do anything except redisplay the cursor;
  . try using the window-start that someone else told us to use
  . try using the same window-start as the previous redisplay, reusing
    as much of the current glyph matrix as possible
  . try using the same window-start as the previous redisplay, after
    correcting it in small ways (like, for example, to move point out
    of the scroll margin)
  . recompute the window-start anew using point as the reference
  . in each case, redisplay the parts of the window using the
    window-start we found

So, as you see, a lot of the processing is dedicated to finding a good
window-start point, which is why if someone told us where to put it,
we prefer to stick to that.  Besides, that someone could be the user
via one of the scroll commands, and in that case the window-start we
were given is mandatory.

> > Yes.  I'm saying that if point is allowed to be outside of the window
> > during some redisplay cycle, we'd need a different criteria for when
> > and how to change w->start.
> 
> Why is that so (for the "how to change w->start" part)?  Making point
> display in the center of the screen in that case will be intuitive for
> most users.

That's not what I had in mind.  What I had in mind was the situation
where point is outside of the window, and the portion of the buffer
shown in the window changes due to something that Emacs does.  If you
are saying that in all such situation we will bring point back into
the window, then my concern is addressed; but if not, we will not be
able to use the final fallback of recentering the window around point,
because that would move the window instead of redrawing the visible
portion of the buffer.