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

[Yuan Fu <casouri@gmail.com>]

> On Oct 19, 2020, at 12:24 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: Yuan Fu <casouri@gmail.com>
>> Date: Mon, 19 Oct 2020 00:59:33 -0400
>> Cc: help-gnu-emacs@gnu.org
>> 
>>>> Cool. I’m thinking about wrapping move_it_by_lines in window_scroll_pixel_based with a version that regards images (and tall lines) as several lines. Do you see some potential problems with that?
>>> 
>>> What is the problem with the existing code that you are trying to
>>> solve?  Whatever it is, it is highly likely that the solution already
>>> exists, and no new code is needed.
>> 
>> The problem I have is that when I scroll over images they jumps in and out of the window: (bad-scrolling.mp4)
> 
> AFAICT, you are trying to scroll an image that is smaller than the
> window?  That's not supported by the current code, you will need to
> modify the criteria for entering the vscroll mode there.
> 
> 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):

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.

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.

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

Yuan