This file is indexed.

/etc/X11/Xsession.d/51gtk3-nocsd-detect is in gtk3-nocsd 3-1.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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# This file is sourced by Xsession(5), not executed.

# Check if /usr/bin/gtk3-nocsd is executable, to make sure that the
# case where gtk3-nocsd is removed but not yet purged is also
# handled properly.
if [ -x /usr/bin/gtk3-nocsd ] && ( [ -z "$GTK3_NOCSD_IGNORE" ] || [ x"$GTK3_NOCSD_IGNORE"x = x"0"x ] ) ; then
    BASESTARTUP=${STARTUP%% *}
    BASESTARTUP=${BASESTARTUP##*/}
    if [ "$BASESTARTUP" = x-session-manager ]; then
        BASESTARTUP=$(basename $(readlink /etc/alternatives/x-session-manager))
    fi
    case "$BASESTARTUP" in
      gnome-session*)
          # This is GNOME, make sure GTK_CSD is not set to 0
          if [ x"$GTK_CSD"x = x"0"x ] ; then
              unset GTK_CSD
          fi
          ;;
      *)
          # not GNOME, and the user didn't specify GTK_CSD
          # themselves, so set it to 0
          if [ -z "$GTK_CSD" ] ; then
              GTK_CSD=0
          fi
          export GTK_CSD
          if [ x"$GTK_CSD"x = x"0"x ] ; then
              export LD_PRELOAD="libgtk3-nocsd.so.0${LD_PRELOAD:+:$LD_PRELOAD}"
          fi
          ;;
    esac
fi