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#79275
30.2.50; overlay line-prefix display property fighting with text display property
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Your message dated Sat, 23 Aug 2025 11:38:56 +0300
with message-id <864ityctzz.fsf <at> gnu.org>
and subject line Re: bug#79275: 30.2.50; overlay line-prefix display property fighting with text display property
has caused the debbugs.gnu.org bug report #79275,
regarding 30.2.50; overlay line-prefix display property fighting with text display property
to be marked as done.
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help-debbugs <at> gnu.org.)
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79275: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=79275
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An overlay which applies a line-prefix (e.g. to set the fringe) across several lines conflicts with underlying text which has a replacing display property set. Evaluate the following in the *scratch* buffer, with at least 3 blank lines at the top of the buffer.
(progn
(delete-all-overlays)
(let ((ov (make-overlay 1 4)))
(overlay-put ov 'line-prefix (propertize "SHOULDNOTSEETHIS" 'display
'(left-fringe right-triangle success))))
(put-text-property 2 3 'display ">testing fringe display"))
The result is very strange: the SHOULDNOTSEETHIS prefix string appears (sans fringe display), but then vanishes on the next redisplay. Some (well-balanced) fight between overlay line-prefix display and normal display properties seems to be occurring.
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> Cc: 79275 <at> debbugs.gnu.org
> Date: Fri, 22 Aug 2025 09:49:04 +0300
> From: Eli Zaretskii <eliz <at> gnu.org>
>
> > From: "J.D. Smith" <jdtsmith <at> gmail.com>
> > Cc: 79275 <at> debbugs.gnu.org
> > Date: Thu, 21 Aug 2025 15:46:03 -0400
> >
> > Eli Zaretskii <eliz <at> gnu.org> writes:
> >
> > > Please try the patch below, I hope it fixes both of the situations you
> > > described.
> >
> > Your latest patch indeed fixes this display conflict between overlays
> > and text properties at both the front of a fringe-displaying overlay and
> > in its middle. The code I used to test:
> >
> > ;; Insert at least 4 blank lines above this and evaluate
> > (progn
> > (delete-all-overlays)
> > (let ((ov (make-overlay 1 5)))
> > (overlay-put ov 'line-prefix (propertize "SHOULDNOTSEETHIS" 'display
> > '(left-fringe right-triangle success))))
> > (put-text-property 1 2 'display ">testing front-fringe display\n")
> > (put-text-property 3 4 'display ">testing mid-fringe display\n"))
> >
> > Thanks very much for your work on this. I know how challenging it can
> > be to hunt down subtle bugs in the redisplay code. I appreciate all the
> > time and energy you volunteer to continuously improve Emacs.
>
> Thanks for testing. I will run a few more tests before installing
> this: as you could see, this bug revealed a couple of serious design
> blunders in how line/wrap-prefix was implemented, so I'd like to make
> sure the changes didn't break anything.
>
> What astonished me the most was the use of it->object to decide
> whether we iterate a buffer or a string, something that I learned long
> ago (and forgot) to be a very bad idea, see this comment in
> dispextern.h:
>
> Do NOT use !BUFFERP (it.object) as a test whether we are
> iterating over a string; use STRINGP (it.string) instead.
>
The fix is now installed on the master branch. Closing the bug.
This bug report was last modified 20 days ago.
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