> ... Couldy you try in
> widget.c and xterm.c to selectively make frame_resize_pixelwise have
> no effect, so we could possibly locate the direct source of the
> problem?  

The attached patch is the minimal patch making the rendering issue
disappear. Also, I cannot reproduce the issue when I try to configure
emacs just with ./configure --with-x-toolkit=lucid. The options I used
to compile emacs in my OS (I am using gentoo) are

./configure --prefix=/usr --build=x86_64-pc-linux-gnu
--host=x86_64-pc-linux-gnu --mandir=/usr/share/man
--infodir=/usr/share/info --datadir=/usr/share --sysconfdir=/etc
--localstatedir=/var/lib --disable-silent-rules
--docdir=/usr/share/doc/emacs-vcs-27.0.9999
--htmldir=/usr/share/doc/emacs-vcs-27.0.9999/html --libdir=/usr/lib64
--program-suffix=-emacs-27-vcs --includedir=/usr/include/emacs-27-vcs
--infodir=/usr/share/info/emacs-27-vcs --localstatedir=/var
--enable-locallisppath=/etc/emacs:/usr/share/emacs/site-lisp
--without-compress-install --without-hesiod --without-pop
--with-dumping=pdumper --with-file-notification=inotify --enable-acl
--with-dbus --with-modules --without-gameuser --with-libgmp --with-gpm
--without-json --without-kerberos --without-kerberos5 --with-lcms2
--with-xml2 --without-mailutils --without-selinux --with-gnutls
--without-libsystemd --with-threads --without-wide-int --with-zlib
--with-sound=alsa --with-x --without-ns --without-gconf
--without-gsettings --without-toolkit-scroll-bars --with-gif --with-jpeg
--with-png --with-rsvg --with-tiff --with-xpm --with-imagemagick
--with-xft --with-cairo --without-harfbuzz --without-libotf
--without-m17n-flt --with-x-toolkit=lucid --with-xaw3d 

Regards,
Ihor