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Re: two display bugs involving interactions between after-string and dis

From: Chong Yidong
Subject: Re: two display bugs involving interactions between after-string and display properties of adjacent overlays
Date: Fri, 29 Feb 2008 13:27:47 -0500
User-agent: Gnus/5.11 (Gnus v5.11) Emacs/22.1.91 (gnu/linux)

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

> BUG #2:  An overlay's display property and after-string property are
> not displayed if an immediately following overlay shares the same Lisp
> string as its display property.  (Using two distinct display strings
> with identical contents works around the bug.)
> Reproduce with this expression:
>   (test-in-fresh-buffer-and-window
>    (insert "ABCD")
>    (let ((o1 (make-overlay 2 3))
>          (o2 (make-overlay 3 4))
>          (s #1=" "))
>      (overlay-put o1 'after-string "1")
>      (overlay-put o1 'display #1#)
>      (overlay-put o2 'display #1#)))
> The above expression should display “A 1 D”.
> The above expression wrongly actually displays “A D”.

This occurs because, as stated in the Emacs Lisp manual, all
consecutive characters that have the same Lisp object as their
`display' property are replaced as a single unit.  In this case, it's
somewhat ambiguous what the behavior should be, but after looking at
the code I think the behavior you suggest would be much more difficult
to implement (and slower) than the current behavior.

Furthermore, you can trivially obtain the behavior you want by making
a copy of the string using copy-sequence, so that the two display
strings are different Lisp objects.

Therefore, let's leave this alone.

Thanks for the report, however.

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