26.3; org-indent-mode's line-prefix text property flickers near overlays

[Kévin Le Gouguec]

Hello,

The line-prefix text property set by org-indent-mode sometimes vanishes
when typing near a line where an overlay is applied.  To reproduce:

- emacs -Q
- C-x C-f repro.org
- M-x org-indent-mode
- M-: (insert "* heading\ntext")
- M-:
(let ((ov (make-overlay (point-at-bol) (point-at-bol)))
      (val (propertize " " 'display '((left-fringe right-triangle)))))
  (overlay-put ov 'before-string val))
- RET
- a

When "a" is inserted, org-indent-mode's line-prefix disappears on the
*previous* line ("text").  It remains gone as long as I type
self-inserting characters, until

- I type certain commands, e.g. RET or C-j, or

- I insert a closing delimiter that makes
  blink-paren-post-self-insert-function blink the corresponding opening
  delimiter, e.g. ')' or ']'.

Then the line-prefix shows up again.


This recipe is simplified; I originally found this bug in Org files
under version control with diff-hl-flydiff-mode enabled.  When typing in
new content, the preceding line is shoved to the left until I stop
typing; after diff-hl-flydiff-delay, diff-hl's idle timer kicks in and
updates its overlay, which as a side-effect makes org-indent-mode's
line-prefix come back up.

See [1] for my original bug report to diff-hl and some crude
"analysis".


I'm clearly out of my depth; at least I hope I used the correct terms to
describe the symptoms I've observed.  Let me know if there is anything I
can do to help debug this.

Thank you for your time.


[1] https://github.com/dgutov/diff-hl/issues/142


In GNU Emacs 28.0.50 (build 21, x86_64-pc-linux-gnu, GTK+ Version 3.24.20, 
cairo version 1.16.0)
 of 2020-05-18 built on my-little-tumbleweed
Repository revision: b1fe27d77db8f819641231ca46725f3eed0b4d9b
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: openSUSE Tumbleweed

Configured using:
 'configure --with-xwidgets --with-cairo'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF
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PDUMPER LCMS2 GMP

Important settings:
  value of $LC_CTYPE: en_US.UTF-8
  value of $LC_TIME: en_GB.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=local
  locale-coding-system: utf-8-unix