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'
Here is again my 'second question'. So after when you do `M-x papyrus-demo`
(and scroll using the arrows) the 'gaps', which are the gray 'gaps' between the pages,
have the height of the default `pixel-line-height`. Setting `papyrus-gap-height` larger
will increase the gap size. However, setting `papyrus-gap-height` smaller will not
decrease the gap size because it seems that the `line-pixel-height` is the minimum.
I would expect the size could be smaller, so my question is why the minimum size is
limited to be `line-pixel-height`...