How to run a fullscreen Python GTK3 application on Debian 9.5 (i386) without a desktop environment installed?

Question

linux

In Debian 9.5 (i386) without a shell installed, the fullscreen method on an object of the Gtk.Window class does not work:

import gi
gi.require_version('Gtk', '3.0')
gi.require_version('Gdk', '3.0')
from gi.repository import Gdk, Gtk, GdkPixbuf


class LockScreen(Gtk.Window):
    def __init__(self, app, config):
        super().__init__(name='lock_screen')
        self.app = app
        cssProvider = Gtk.CssProvider()
        cssProvider.load_from_path('./static/styles.css')
        screen = Gdk.Screen.get_default()
        styleContext = Gtk.StyleContext()
        styleContext.add_provider_for_screen(screen, cssProvider, Gtk.STYLE_PROVIDER_PRIORITY_USER)
        self.box = Gtk.Box(halign=Gtk.Align.CENTER, orientation=Gtk.Orientation.VERTICAL, parent=self,
                           valign=Gtk.Align.CENTER, name='lock_screen_box')
        self.tonb_label = Gtk.Label('Тюменская Областная Научная Библиотека', justify=Gtk.Justification.CENTER,
                                    name='tonb_label', parent=self.box)
        self.icons = {
            name: GdkPixbuf.Pixbuf.new_from_file_at_scale('./static/icons/{0}.svg'.format(name), 96, 96, True)
            for name in [
                'app_started',
                'network_connected',
                'network_disconnected',
                'workstation_locked',
                'workstation_unlocked',
                'workstation_shutdown',
                'reader_binded',
                'error',
            ]
        }
        self.status_icon = Gtk.Image(name='status_icon', parent=self.box)
        self.status_label = Gtk.Label(justify=Gtk.Justification.CENTER, name='status_label', parent=self.box,
                                      width_chars=56)
        self.fullscreen()
        self.connect("destroy", Gtk.main_quit)
        self.show_all()

    ...

The Python Gtk3 application window gets the minimum size:

However, freerdp runs in full screen mode (-f switch) as it should:

In addition, everything works as it should in Linux Mint 18.3 Cinnamon, the Python Gtk3 application runs in full screen mode without problems.
Packages installed on Debian to run Gtk3:
xorg python3-gi python3-click python3-gi-cairo python3-cairo gir1.2-gtk-3.0 libglib2.0-dev libcairo2-dev libgirepository1.0-dev
The application is launched via lightdm.
/etc/lightdm/lightdm.conf
greeter-session=defender_greeter
/usr/share/xgreeters/defender_greeter.desktop

[Desktop Entry]
Name=TONB Thin Defender
Comment=No comments
Exec=/bin/bash /home/defender/thin-defender/thin_defender/run.sh
Type=Application
X-Ubuntu-Gettext-Domain=lightdm

/home/defender/thin-defender/thin_defender/run.sh

cd /home/defender/thin-defender/thin_defender
/home/defender/thin-defender/.venv/bin/python3 /home/defender/thin-defender/thin_defender/manage.py run_app --config test

UPDATE
A number of Gtk.Window methods require a window manager to work correctly, including the fullscreen method.
Possible solutions to the problem: try to do without these methods or install some kind of window manager.

Answer 1

I tried to install and configure openbox as a window manager, but got tired of the configs.
It turned out to be easier not to use the fullscreen() method, which depends on window managers:
But to get the current display width and height from Gdk.Screen and set it using the resize() method:
Result:

Answer 2

Without what "shell"? Without X? Nothing graphic will ever start without them :)