help understanding overlay behaviour with images

help understanding overlay behaviour with images

[David Bremner]

I'm not sure if this is a bug or if I misunderstand something.  In the
following code sample I expect the image to be hidden, but it isn't,
unless I also hide the previous character, i.e. pass 10 as the first
argument to make-overlay. In both Emacs 28.2 and 29.1 (tested on
Debian), it gets some weird in between state where the point skips over
the image when running left-char or right-char, but the image stays visible.
  
  (let ((buf (get-buffer-create "image-buffer"))
        (img (find-image '((:type xpm :file "attach.xpm"))))
        (overlay nil))
    (switch-to-buffer buf)
    (insert "0123456789")
    (insert-image img "x")
    (insert "0123456789")
    (insert "\n")
    (setq overlay (make-overlay 11 12))
    (overlay-put overlay 'invisible t)
    (message "props=%s" (overlay-properties overlay)))

P.S. I'm not subscribed to emacs-devel, so please CC me any answers.

Re: help understanding overlay behaviour with images

[Eli Zaretskii]

> From: David Bremner <david@tethera.net>
> Date: Wed, 16 Aug 2023 07:45:12 -0300
> 
> I'm not sure if this is a bug or if I misunderstand something.  In the
> following code sample I expect the image to be hidden, but it isn't,
> unless I also hide the previous character, i.e. pass 10 as the first
> argument to make-overlay. In both Emacs 28.2 and 29.1 (tested on
> Debian), it gets some weird in between state where the point skips over
> the image when running left-char or right-char, but the image stays visible.
>   
>   (let ((buf (get-buffer-create "image-buffer"))
>         (img (find-image '((:type xpm :file "attach.xpm"))))
>         (overlay nil))
>     (switch-to-buffer buf)
>     (insert "0123456789")
>     (insert-image img "x")
>     (insert "0123456789")
>     (insert "\n")
>     (setq overlay (make-overlay 11 12))
>     (overlay-put overlay 'invisible t)
>     (message "props=%s" (overlay-properties overlay)))

This is expected: a 'display' property (which is how Emacs implements
display of images in a buffer) causes Emacs to ignore the text
"covered" by the property, in this case the character "x" on which you
placed the overlay with the invisible property.  So Emacs doesn't see
the overlay on "x", and doesn't act upon the invisible property of
that overlay.

IOW, you have here two display features, each of which needs to skip
the same buffer positions, so the first one wins.