bug#26153: 26.0.50; Garbage displayed at visual line boundaries

[Yuri D'Elia <wavexx@thregr.org>]

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In the last weeks a new issue crept up in the display engine. Some
visual garbage is drawn at the horizontal lower/upper edge when using
hl-line-mode, on the region, or anything that redraws the current visual
line to fill the background.

See the attached screenshot, captured with emacs -Q on the current
master, with the lucid toolkit.

Evaluate the test file, then move the pointer down one line at a time,
until garbage is visible.

Changing the font weight seems necessary to trigger this issue, as well
some hidden properties in the line. I wasn't able to reproduce it easily
otherwise.

This seems to have been introduced around commit
d546be31a9320d94769cb322f008f49d08d852a8 ("Fix display of
mouse-highlight produced by overlapping overlays") or earlier.

If the exact commit would be helpful, I can try to pinpoint it.


Configured using:
 'configure --with-x-toolkit=lucid --without-gconf --without-gsettings
 --with-modules 'CFLAGS=-O3 -march=native -pipe '
 LDFLAGS=-fwhole-program'

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND DBUS NOTIFY ACL LIBSELINUX GNUTLS
LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS LUCID X11
MODULES LIBSYSTEMD

Important settings:
  value of $LC_COLLATE: C
  value of $LC_TIME: en_DK.UTF-8
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix