Re: anomalies of overlays and before-string and display properties

[Joe Wells <jbw@macs.hw.ac.uk>]

By the way, with one exception, these problems also occur with
after-string properties.

The exception is that the overlay's mouse-face property does correctly
cover both the overlays display and after-string properties.  This is
more evidence that not covering the before-string property is a bug.

Strangely, the after-string also “inherits” face attributes from the
character after the beginning of the overlay, not the character just
before the _end_ of the overlay.  This is more evidence that this is a
bug and not a feature.

Joe

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

> There are a number of strange anomalies (some clearly bugs) in the
> behavior of overlays and before-string and display properties.
>
> 1. Faces on a display property on a before-string behave differently
>    depending on where the display property is in the before-string.
>    For a display property at the beginning of the before-string, its
>    face is used and the before-string's face is ignored (I think this
>    is correct).  For a display property not at the beginning of the
>    before-string, its face is completely ignored, and the face of the
>    before-string is used instead.  This is fairly clearly a bug.
>
> 2. When a substring of a before-string beginning at offset O has a
>    display property which is a string S, the first O characters of S
>    are not displayed.  If S is not at least O+1 characters long, then
>    disastrous things start to happen.  This is clearly a bug.
>
> 3. The before-string somehow “inherits” face attributes from the face
>    of the character that follows the start of the overlay.  (If the
>    overlay is of length zero, this character is not even in the
>    overlay.  See the recent discussion of how this problem is
>    affecting linum.el.)  This means that in practice a before-string
>    needs to use completely specified faces, which is obviously a real
>    pain.
>
> 4. The display property of an overlay somehow “inherits” face
>    attributes from the face of the character that follows the start of
>    the overlay.  This has similar disadvantages to anomaly #3
>    mentioned above.
>
> 5. The help-echo and mouse-face properties of an overlay with both a
>    before-string and a display property only affect the material
>    displayed by the overlay's display property.  One can work around
>    the issue by copying these properties to the various display
>    properties inside the before-string; however, this workaround is a
>    pain and in the case of mouse-face it doesn't get the entire
>    overlay highlighted at once but instead only the pieces are
>    highlighted one at a time.  It would be nice to be able to set a
>    single help-echo or a single mouse-face property that would affect
>    the _entire_ overlay.
>
> You can reproduce all of these anomalies with this function:
>
>   (defun illustrate-anomalies ()
>     (let ((s #1=#("\\abcd{VWXYZ}"
>                   0 5 (face (:foreground "Purple" :background "yellow"))
>                   5 12
>                   (face
>                    (:foreground
>                     "DarkOliveGreen"
>                     :weight bold :slant oblique :height 0.8))))
>           (props
>            '(help-echo
>              #1#
>              ;;face (:slant italic :foreground "red")
>              display "]"
>              before-string
>              #("  "
>                0 1
>                (display
>                 #("|["
>                   0 2 (;; help-echo #1#
>                        face #2=(:strike-through t :background "white")))
>                 ;; help-echo #1#
>                 face #3=(:underline t))
>                1 2
>                (display
>                 #("||VWXYZ"
>                   0 7 (;; help-echo #1#
>                        ;; mouse-face (:background "darkseagreen2")
>                        face #2#))
>                 ;; help-echo #1#
>                 face #3#))
>              mouse-face (:background "darkseagreen2")))
>           (buf (get-buffer-create "xyzzy")))
>       (with-current-buffer buf
>         (display-buffer buf)
>         (erase-buffer)
>         (dolist (o (overlays-in (point-min) (point-max)))
>           (delete-overlay o))
>         (insert s)
>         (let ((o (make-overlay (point-min) (point-max))))
>           (while props
>             (overlay-put o (car props) (cadr props))
>             (setq props (nthcdr 2 props)))))))
>
> This example is derived from some work where I was trying to get
> AUCTeX's folding mode to show the fontified and latex-previewed
> contents of macro arguments.
>
> I hope this problem report is useful.
>
> Joe
>
> ======================================================================
> In GNU Emacs 22.1.1 (i686-pc-linux-gnu, GTK+ Version 2.8.20)
>  of 2007-06-27 on artemis
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