Re: Set-window-vscroll sometimes doesn't work

[Yuan Fu]

> What do you mean by "smooth scrolling" here?  If I invoke C-2 C-n" on
> a large image, the image gets scrolled by the amount of pixels that is
> equivalent to 2 screen lines.  Isn't that what you see?

With emacs -Q? On my machine emacs -Q doesn’t to that. Instead of moving two 
line’s height, it just jumps to the next logical line.

> When vscroll is so large that the window-start point is entirely not
> visible, you should move window-start so it becomes visible.  But that
> shouldn't be a problem, because it means you've scrolled the entire
> image off-screen.

> On Oct 22, 2020, at 12:08 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: Yuan Fu <casouri@gmail.com>
>> Date: Thu, 22 Oct 2020 00:16:04 -0400
>> Cc: help-gnu-emacs@gnu.org
>> 
>> Ok, I looked at redisplay_window, and it zeros out vscroll if force_start is 
>> true, that explains my initial
>> question. After reading the function, it seems that setting window-start is 
>> the primary way to scroll and to
>> produce the glyph matrix to display. IIUC this is how window-start and 
>> vscroll works together in window
>> redisplay: we have a window-start -> try window -> start_display on 
>> window-start -> it_initialize on
>> window-start -> it->current-y adjusted according to vscroll -> back to try 
>> window -> while (it.current_y <
>> it.last_visible_y): produce glyph rows. This doesn’t seem to be too 
>> contradictory, and scrolling by setting
>> window-start and add sub-line adjustment with vscroll seems to be complaint 
>> with the redisplay logic. What
>> am I missing?
> 
> You are missing the use cases where the display element that is taller
> than the normal text is not an image, but something else.  For
> example, text displayed with a very large font.  In this other use
> case, starting display with a non-zero vscroll when the Lisp program
> forced window-start will produce text part of which cannot be read,
> because too much of the characters is off-screen.
> 
> This is why setting window-start zeroes out vscroll: Emacs wants to
> make sure that in this case the entire screen line that starts at
> window-start will be fully visible.
> 

That’s a fair point. But what about scrolling down into an image? I’ll need to 
set window-start on the image then set vscroll to image height - line height so 
it only displays the bottom strip. That’s what line-move does, too:

(prog1 (line-move-visual arg noerror)
            ;; If we moved into a tall line, set vscroll to make
            ;; scrolling through tall images more smooth.
            (let ((lh (line-pixel-height))
                  (edges (window-inside-pixel-edges))
                  (dlh (default-line-height))
                  winh)
              (setq winh (- (nth 3 edges) (nth 1 edges) 1))
              (if (and (< arg 0)
                       (< (point) (window-start))
                       (> lh winh))
                  (set-window-vscroll
                   nil
                   (- lh dlh) t))))

Yuan