bug#57066: 29.0.50; Complex emoji overlaps with text

[Protesilaos Stavrou]

Dear maintainers,

I just noticed a bug with some complex emoji (grapheme clusters?) where
the emoji does not have clear character boundaries but instead overlaps
with text adjacent to it.

Steps to reproduce on 'emacs -Q':

* In the scratch buffer type "Hello "
* C-x 8 e s
* Search for "sun" and type RET
* Now you have: Hello ☀️
* C-e
* Type something else, like "test".
* Notice how "test" is written over the emoji (see attached screenshots).

Please let me know if you need me to test anything else on my side or
provide additional information.

All the best,
Protesilaos (or simply "Prot")

* * *

In GNU Emacs 29.0.50 (build 3, x86_64-pc-linux-gnu, GTK+ Version 3.24.34, cairo 
version 1.17.6)
 of 2022-08-09 built on kronos
Repository revision: 97a54d132732131fb7635d91a45cf2088e98b60b
Repository branch: makepkg
System Description: Arch Linux

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --mandir=/usr/share/man --with-gameuser=:games
 --without-libotf --without-m17n-flt --without-gconf
 --with-native-compilation --with-pgtk --with-sound=no --without-gpm
 --without-compress-install
 '--program-transform-name=s/\([ec]tags\)/\1.emacs/'
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fno-plt -fexceptions
 -Wp,-D_FORTIFY_SOURCE=2 -Wformat -Werror=format-security
 -fstack-clash-protection -fcf-protection'
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LCMS2 LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PGTK
PNG RSVG SECCOMP SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS WEBP XIM GTK3
ZLIB

Important settings:
  value of $LANG: C
  locale-coding-system: nil

-- 
Protesilaos Stavrou
https://protesilaos.com

Screenshot from 2022-08-09 12-07-53.png
Description: PNG image

Screenshot from 2022-08-09 12-08-20.png
Description: PNG image