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

[Yuan Fu]

> 
>>>> That is why I suggest to stick to the code in simple.el as much as
>>>> possible: it is well tested and works, AFAIU you need to change it
>>>> very little to be able to scroll images that are smaller than the
>>>> window height -- that test is in a small number of places and can be
>>>> replaced by something else.
>>>> 
>>> 
>>> next-line and friends only vscroll when scroll-conservatively=0 and ARG=1, 
>>> which allows it to get away from the problem I face (set-window-start and 
>>> set-window-vscroll in the same time). I either implements smooth scrolling 
>>> to only work when ARG=1 or I need to set both value at the same time.
>> 
>> I don't understand why you say this.  First, C-n works on tall images
>> with arguments greater than 1.  And second, I still don't understand
>> why you need to set window-start, because vscroll should be enough for
>> your needs, as far as I understand.
> 
> C-n doesn’t smooth scroll on tall images when ARG > 1: The code checks for 
> ARG = 1, if not, it doesn’t do line-move-partial. I also tried C-2 C-n, no 
> smooth scrolling.
> 
> I thought vscroll is a relative measure and is measured against window-start. 
> And it kind of makes sense to set a window start as the basis and set vscroll 
> based on that. So I kind of assumed that window-start is always at least 
> partially visible and vscroll is always less than the height of the first 
> line. Now, if I throw away window-start and scrolls entirely by vscroll, will 
> there be any problems? Like, does anyone relies on window-start being 
> visible? I felt like that’s what people uses to get the first line displayed 
> in the window.
> 

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?

Yuan