display property as display spec vector behavior unclear

display property as display spec vector behavior unclear

[Joe Wells]

The documentation in the Emacs Lisp manual on the display property
states this:

  The value of the `display' property should be a display
  specification, or a list or vector containing several display
  specifications.

Unfortunately, this is the only thing it states about the case where
the display property is a list or vector of display specs.
I searched for all occurrences of the words “list” and “vector” in
that section of the manual and could not find more precise details.

I would guess from the above description that if the display property
were a vector of display specs, then all of the things specified to be
displayed would be shown from left to right.  However, it seems that
only the first one is used, which seems strange.

Reproduce by evaluating this code:

  (let ((buf (get-buffer-create "foo")))
    (with-current-buffer buf
      (display-buffer buf)
      (erase-buffer)
      (dolist (o (overlays-in (point-min) (point-max)))
        (delete-overlay o))
      (insert "ABC")
      (let ((o (make-overlay 2 3)))
        (overlay-put o 'display ["1" "2" "3"]))))

You will see the “foo” window pop up and in the window you will see
“A1C”.  I expected that instead “A123C” would be shown.

If this is a documentation bug, could the documentation please be
clarified?  If this is a code bug, could the code please be fixed?

By the way, the code in handle_display_prop in xdisp.c does explicitly
loop over all of the elements of the vector.  So the code is going to
the effort to do something with all of them, although only the work
from the first one is currently showing up.

Also by the way, it would be *extremely* useful if all of the display
specifications in a vector were shown.  I'm trying to make the
features of AUCTeX's tex-fold.el work together with latex-preview, and
if a vector of display specs were all shown this would make things
really easy.

Thanks for your time in considering this issue.

Joe

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Re: display property as display spec vector behavior unclear

[Johan Bockgård]

Joe Wells <jbw@macs.hw.ac.uk> writes:

> The documentation in the Emacs Lisp manual on the display property
> states this:
>
>   The value of the `display' property should be a display
>   specification, or a list or vector containing several display
>   specifications.
>
> Unfortunately, this is the only thing it states about the case where
> the display property is a list or vector of display specs.
> I searched for all occurrences of the words “list” and “vector” in
> that section of the manual and could not find more precise details.
>
> I would guess from the above description that if the display property
> were a vector of display specs, then all of the things specified to be
> displayed would be shown from left to right.  However, it seems that
> only the first one is used, which seems strange.

A vector or list is used for combining properties (where is makes
sense), not laying things out in sequence.

This passage from the comment of handle_single_display_spec is
relevant:

    DISPLAY_REPLACED_P non-zero means that we previously saw a
    display specification which already replaced text display
    with something else, for example an image; we ignore such
    properties after the first one has been processed.

Properties that don't "replace text with something else" include
`height', `raise', `space-width'. These can be combined.

Image slices use `slice' together with `image'.

-- 
Johan Bockgård