B.t.w to see how things work when adding the `set-window-start`,
you could uncomment that function in the `image-roll-next-page`
function. You will find that the `image-set-window-vscroll` at the
end of the `image-roll--redisplay` function has no effect.

To see how things work when adding the redisplay, you could
additionally uncomment it at the end of the `image-roll--redisplay`
function.


On Thu, 28 Apr 2022 at 18:28, dalanicolai <dalanicolai@gmail.com> wrote:

> Thank you Po, for your explanations. Indeed this clears things
> up a little, but unfortunately my question was not 'precise enough',
> for the answer to 'solve' the issue that motivated this question.
> (I wanted to keep my `image-mode.el` out of the question, but I
> guess it is necessary to add it to explain the issue I am experiencing).
>
> So I have rewritten `image-mode.el` to scroll by `line + vscroll` instead
> of using vscroll only. This solves the issue with the RAM usage for
> 'large documents'.
> Additionally, it made the code a little 'simpler', which is a welcome
> 'side effect'
> (I got rid of the 'gap' (page separation) overlays, and instead added
> margins
>  to the images. So now the buffer contents is just single spaces  on
> separate
> lines, where the number of lines is equal to the number of `images/pages`.
> The 'gap' color can be configured by adding the overlay's face background).
> It seems to work mostly correct (the 'scrolling' look a tiny bit less
> smooth than
> with the pure vscroll version, but I would say it is very acceptable).
>
> Anyway, I hope someone can quickly have a look at the issue explained below
> (this should take less than a minute). So let me quickly provide some
> context
> for the issue.
>
> Currently, each overlay is overlaid on a single space on separate lines.
> So the
> contents of the buffer is a single space (holding a page overlay) on every
> line.
> So to jump to the next page, I can simply use 'forward-line'. Now this
> works fine
> when the page is 'off-window` but when the next page is already within view
> ('on-window'), then going to the next line does not position that line at
> the top of
> the window, nor makes it the current `window-start. To get it at the top
> of the
>  window, I could subsequently call `(set-window-start nil (point))`,
> but then subsequently, `set-window-vscroll` does not work. To make
> `set-window-vscroll` have effect, I can add a `redisplay` just before I
> call it.
> This works, but the 'jump' gets very/unacceptably ugly (you see the
> 'recentering'
> steps taking place).
>
> However, when the next page is 'off-window' than using the 'forward-line'
> works
> directly, and the page jump looks smooth.
>
> So to reproduce it from `emacs -Q` you can just load the `image-roll.el`
> file that I
> will attach here, then do `M-x image-roll-demo` and scroll down using the
> arrow
> keys until you see the page transition between page one and two. Now press
> the
> `page down` (PgDn) button to jump to the next page.
>
> Because page two is already 'on-window' this only moves the cursor one
> page down.
>
> If you now press a second time `PgDn` then, because page 3 is still
> `off-window`,
> the jump takes place how it should.
>
> I would be happy, if someone could tell me how I could 'solve' this issue.
> Otherwise,
> all scrolling functions work fine, although I did not yet take care of the
> 'edge cases'
> (i.e. first and last page). Also, I did not update the docstrings yet, so
> I am not asking
> for feedback on those. My only question is if someone could have a look at
> what I am
> describing above.
>
> On Thu, 28 Apr 2022 at 14:07, Po Lu <luangruo@yahoo.com> wrote:
>
>> dalanicolai <dalanicolai@gmail.com> writes:
>>
>> > I have a question about this, namely: how to make a line the 'window
>> > start'?  Using 'set-window-start` does not work.
>> >
>> > From 'emacs -Q' (which starts within the scratch buffer), immediately
>> > evaluate
>> >
>> > (set-window-start nil (point))
>> >
>> > to set the 'new' window start. Subsequently do
>> >
>> > (set-window-vscroll nil 1)
>> >
>> > it will scroll from the start of the buffer, and not from the 'new'
>> > window start as I would expect
>>
>> That's because the point is obscured after the vscroll is applied, so
>> the display is recentered.  You have to move point to some location that
>> is at least partially visible after the vscroll if you set
>> `make-cursor-line-fully-visible' to t, or a location that is fully
>> visible otherwise.
>>
>