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

[Yuan Fu]

> On Oct 18, 2020, at 1:55 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: Yuan Fu <casouri@gmail.com>
>> Date: Sun, 18 Oct 2020 13:51:14 -0400
>> Cc: help-gnu-emacs@gnu.org
>> 
>>> NOFORCE non-nil means Emacs doesn't have to display the window
>>> starting at the position if there are valid reasons, so yes.  But my
>>> suggestion is not to use set-window-start when you want to vscroll,
>>> it's unsafe.  You in effect create a conflict for the display engine,
>>> and have no control on how it will resolve that conflict.
>> 
>> Thanks. I see what you mean. What I want to do is to scroll over images as 
>> if it is made of several lines (like sliced images). Which requires 
>> scrolling over logical lines by set-window-start and scrolling over images 
>> by set-window-vscroll. Do you think I would be better off implementing this 
>> in, say, window_scroll_pixel_based rather than in Lisp?
> 
> It sounds like you want to do what we already have implemented in
> next-line and previous-line and their subroutines.  Typing C-n on a
> tall image scrolls the image the number of pixels that is equal to the
> line height.  Maybe you can just use that code?
> 

Some questions:

1. Is it true that move_it_by_lines regards an image as one display line?
2. It seems that when I move focus out of the window, the vscroll setting is 
nullified, why is that?

Yuan