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

[Eli Zaretskii]

> From: Yuan Fu <casouri@gmail.com>
> Date: Mon, 19 Oct 2020 12:56:38 -0400
> Cc: help-gnu-emacs <help-gnu-emacs@gnu.org>
> 
> > But in any case, I don't see how move_it_by_lines can have anything to
> > do with this: whatever you do, an image must always remain a single
> > "display element", and the only way we currently have to scroll
> > partial images is via vscroll.  Which works, so I don't understand why
> > you need any changes in window_scroll_pixel_based.
> > 
> 
> What I came up with is this:
> 
> For (scroll-up 1):

That's your problem, right there: scroll-up is not the right command
for this.  Scroll commands work by setting window-start, and you don't
want that with large images.  They also move by window-full, again
something that is not appropriate for scrolling images.  You'd need a
different command for your use case.

Please read the code in simple.el, the functions line-move,
line-move-1, and line-move-partial -- those perform scrolling of tall
images when needed.  You will find there how to decide when to set
vscroll and to what value; I suggest to use a similar strategy.

> 
> 1. If the first row is a normal line (i.e., with normal height), scroll up 
> normally by one display line.
> 2. If the first row is an image, adjust vscroll += 
> frame-default-pixel-line-height
> 3. If the first row is an image and we have scrolled to the bottom of the 
> image, i.e., vscroll = image-height, scroll up one display line and set 
> vscroll to 0.

You can do it this way, but I think it will be confusing because
scroll-up is rarely used with an argument of 1.

But if you insist on doing this inside scroll-up, then items 2 and 3
should do what line-move-visual etc. in simple.el do when they need to
scroll a tall image.  They do exactly what you describe above (they
also do other stuff, which you can remove if you want).

> In other words, sometimes we are not really scrolling, but faking a scroll by 
> adjusting vscroll. IIUC window_scroll_pixel_based moves it by 
> move_it_by_lines and set window-start to it. That’s not what we want when we 
> want to fake scroll by adjusting vscroll: in that case we don’t want to 
> change window-start.

Indeed, that's not what we want.  Which is why scroll commands are not
the right starting point for what you want to do.

> 1. Is there a way to get the line-height that an it is on? 

Yes, use line-pixel-height.  Again, see the functions in simple.el I
pointed to, they do it already.

> 2. Where is the code that ensures point is always fully visible? When I move 
> point around Emacs adjusts scrolling to keep the point fully visible, but I 
> don’t know who’s doing that work.

It's in the display engine (xdisp.c).  Many places there check whether
the screen line of point is fully visible and retry redisplay if not.