bug#62048: 30.0.50; Non-nil `line-spacing' takes precendence over 'line-height t text property

[Eli Zaretskii <eliz@gnu.org>]

> From: Ihor Radchenko <yantar92@posteo.net>
> Date: Wed, 08 Mar 2023 12:18:59 +0000
> 
> According to 41.11 Line Height section of Elisp manual,
> 
>        A newline can have a ‘line-height’ text or overlay property that
>     controls the total height of the display line ending in that newline.
>     The property value can be one of several forms:
>     
>     ‘t’
>          If the property value is ‘t’, the newline character has no effect
>          on the displayed height of the line—the visible contents alone
>          determine the height.  The ‘line-spacing’ property, described
>          below, is also ignored in this case.  This is useful for tiling
>          small images (or image slices) without adding blank areas between
>          the images.
> 
> AFAIU, it implies that buffer-local `line-spacing' value is also
> ignored.
> 
> Consider the following minimal example:
> 
> 1. emacs -Q
> 2. (setq-default line-spacing 0.15)
> 3. (with-silent-modifications (insert-sliced-image (create-image "/path/to/some/image") nil nil 10 nil))
> 
> Expected: the spliced image is displayed without gaps.
> Observed: visible gaps between image rows
> 
> If I understand correctly, `insert-sliced-image' tries to avoid the gaps
> by (insert (propertize "\n" 'line-height t)). But it does not seem to be
> working.
> 
> Am I missing something?

The above text in the manual is misleading: only the line-spacing
property of the same newline is ignored in this case.  (It makes no
sense to ignore the buffer-local value of line-spacing due to a single
newline, because the buffer-local value of line-spacing affects all
the lines in the buffer, not just one line.)  IOW, the line-height
property of the newline overrides the 'line-spacing' property of the
same newline.

I've updated the manual to be more accurate about this.  But other
than that, this is not a bug in the code.