Re: Redisplay issues with 'space' display properties on overlays

[Eli Zaretskii <eliz@gnu.org>]

> From: dalanicolai <dalanicolai@gmail.com>
> Date: Sat, 16 Apr 2022 18:37:24 +0200
> 
> 1. papyrus.el uses the `papyrus-redisplay` function, which creates the 'scroll'
> by adding 'space' display properties to the overlays, and page images to
> the 'currently visible overlays'. The redisplay works fine on Spacemacs/Doom,
> but for vanilla Emacs, in order to 'finalize' the process requires a hack which is
> to 'print' something to the echo area, after which the 'redisplay' will finish
> and show the images.
> 
> I have tried to use `redisplay/force-window-update` for this, but those do not
> seem to have the right effect.
> 
> Obviously, my question is if you can tell me why the hack 'is required' and what
> would be the better way to achieve/finalize the redisplay.
> 
> 2. I would expect that a specified space (space display property for the 'gap'
> overlays, i.e. the space between the pages) could be smaller than the default
> 'line-pixel-height`. However, this does not seem possible by just setting a
> smaller :height for the space display property, as the overlay will always have
> a minimum height of the default 'line-pixel-height'
> 
> To see/reproduce what I mean you can simply load `papyrus.el` and do
> `M-x papyrus-demo`. You can experiment with the 'gap/space-height' by setting
> the `papyrus-gap-height`.
> 
> For 'reproducing' the 'issue' of the first question, you can simply load
> pdf-scroll.el (but first set the correct path to `papyrus.el` at the beginning
> of that file. Also, pdf-scroll.el requires pdf-tools to be correctly installed).
> Subsequently, open some pdf file, and split the window. I hope you will find
> that the continuous scroll, including the functionality of the
> `pdf-tools-enabled-modes`, works perfectly fine in vanilla Emacs (scroll with arrows
> and page-down/up). Now, to see what I mean with the display 'issue' comment out the
> last two lines of the `papyrus-redisplay` function, and again try to split the
> window from the pdf buffer, you will find that the extra step of printing is
> required before the image will show correctly.

Since using the actual code requires complex setups, would you please
show simpler recipes for these two issues, ones that can be used
easily from "emacs -Q"?  Then I will try to answer your questions
above.

Thanks.