Kivy, Python: Layout with nested rules produces no output of child widgets

Question

I have a simple App, that is supposed to use a ScreenManager for different screens. When I create one of my Screens and make it the app.root widget everything works fine. But as soon, as I put my Screen as a child inside the ScreenManager, it produces no output as if the widgets were not there at all.

testScreenManager.py:

from kivy.app import App

from kivy.uix.textinput import TextInput
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button
from kivy.graphics.vertex_instructions import Rectangle
from kivy.uix.screenmanager import ScreenManager, Screen

class MyScreenManager(ScreenManager):
    pass

class myScreen(Screen):
    pass

class myNavBoxes(BoxLayout):
    pass

class myApp(App):
    def build(self):
        return MyScreenManager()

if __name__ == "__main__":
    myApp().run()

my.kv:

<MyScreenManager>:
    canvas.before:
        Rectangle:
            pos: self.pos
            size: self.size
    myScreen:

<myScreen>:
    BoxLayout:
        orientation: 'vertical'
        size_hint: 0.8, 0.3
        pos_hint: {'x':0.1, 'y':0.5}
        TextInput:
            id: username
            hint_text: 'Username'
            font_size: '20sp'
    myNavBoxes:

<myNavBoxes>:
    orientation: 'horizontal'
    size_hint: 1, .2
    pos_hint: {'x':.0, 'y':.0}
    Button:
        id: navButtonLogin
        text: 'Login'
        font_size: '15sp'
        size_hint_y: None

This produces a blank white rectangle with no widgets inside it.

I tried a lot of things. One main concern is this: If I don't use custom widgets (which inherit directly from normal kivy-classes) and use the base kivy-classes instead, the code produces expected output:

<MyScreenManager>:
    canvas.before:
        Rectangle:
            pos: self.pos
            size: self.size
    Screen:
        BoxLayout:
            orientation: 'vertical'
            size_hint: 0.8, 0.3
            pos_hint: {'x':0.1, 'y':0.5}
            TextInput:
                id: username
                hint_text: 'Username'
                font_size: '20sp'
        myNavBoxes:


<myNavBoxes>:
    orientation: 'horizontal'
    size_hint: 1, .2
    pos_hint: {'x':.0, 'y':.0}
    Button:
        id: navButtonLogin
        text: 'Login'
        font_size: '15sp'
        size_hint_y: None

Answer 1

The problem is your class names. Make all your class names start with upper case letter. The kv file is sensitive to this.