Let me, additionally, very briefly explain what the code is doing, and
what I am expecting (as an attempt to make it immediately clear what I
mean).

So the code is first simply creating (for my dir) 108 overlays.  Then
in 'scrap-dir-images' are made to display the images from the
directory. Immediately, after the
'(seq-do-indexed (lambda (im n) (overlay-put ... 'display ...)'
the function print the value of '(overlays-in (window-start) (window-end))',
which return 108. However, I expect it to print 2, as the overlays should
have the size of the images now (and I see only two images simultaneously on
screen). Indeed when doing 'M-: (overlays-in (window-start) (window-end))'
immediately after creating the 'image gallery', it returns 2.

On Tue, 21 Feb 2023 at 14:39, dalanicolai <dalanicolai@gmail.com> wrote:

> In this example code, I am simply displaying all images at once,
> because I am assuming that the images in the directory are not too
> many and that they are small (which is not a very reasonable
> assumption, but this is just my personal 'test' function).
>
> Indeed, the example does not call sit-for, because it should show that
> the printed number of overlays, is the number of all images in the
> directory (instead of only the number of images currently on the
> screen, i.e. within '(overlays-in (window-start) (window-end))' ).
>
> For example, I have a directory with 108 images, when I run 'M-x
> scrap-dir-images' it print 108, although it should print the number of
> images on screen i.e. by '(overlays-in (window-start) (window-end))',
> which value is what 'scrap-dir-images' prints.  However, the images
> are normal 'foto' size, so that I only see two images on screen, and
> indeed doing 'M-: (overlays-in (window-start) (window-end))' manually
> now prints 2.  If I had added the '(sit-for)' in the example
> code, then I would have 'fixed' the problem, and 'scrap-dir-images'
> would have printed 2 immediately because the display property
> ('expansion of the overlays') got enough time to take effect.
>
> I hope this clears things up, but of course I would be happy to try
> another explanation (e.g. sending by adding an animated gif).
>
> But this function is a no-op in Emacs 29 and later, since the overlays
>> were reimplemented in a way that makes it unnecessary to "center" the
>> list of overlays.  So you can forget about that and ignore this
>> function.
>>
>
> Thanks, that is indeed handy to know.
>
> On Tue, 21 Feb 2023 at 14:21, Eli Zaretskii <eliz@gnu.org> wrote:
>
>> > From: dalanicolai <dalanicolai@gmail.com>
>> > Date: Tue, 21 Feb 2023 13:46:56 +0100
>> >
>> > So now here are the questions:
>> >
>> > - when displaying multiple pages in columns, I would like to use the
>> > overlays-in function to determine which overlays should display
>> > images.  So I am creating a full 'book roll' by giving the overlays a
>> > size via the 'space' display property, after which I use overlays-in
>> > to determine which overlays are actually visible. However, after it
>> > takes some time for the 'space' display property to take effect, so I
>> > am manuall adding a 'sit-for' with some reasonable delay
>> > time. However, I would like to ask if there is someone has an idea for
>> > a 'better' mechanism to wait until/detect if the 'overlay expansion'
>> > has finished.
>> >
>> > If the explanation is not clear then please load the following file
>> > and do 'M-x scrap-dir-images' on a directory that contains enough
>> > images to not fit all on a single screen. It will print the number of
>> > overlays found via 'overlays-in' directly after 'displaying the
>> > images' (here by assigning the image as display property instead of
>> > space). You will find it prints all overlays in the buffer (instead of
>> > only the ones on screen). To find what I expect it to print now
>> > (again) do 'M-: (overlays-in (window-start) (window-end))'.
>>
>> I did all that, and I still don't understand the question.  In
>> particular, your code doesn't call sit-for, so I'm unsure what exactly
>> is the problem you are asking about here.
>>
>> > My second question is about the function 'overlay-recenter' I don't
>> > really understand its docstring. What kind of 'overlay lookup' would
>> > go faster? What is 'overlay-lookup' anyway?
>>
>> Looking up overlays that are relevant to a particular buffer position.
>>
>> But this function is a no-op in Emacs 29 and later, since the overlays
>> were reimplemented in a way that makes it unnecessary to "center" the
>> list of overlays.  So you can forget about that and ignore this
>> function.
>>
>