Kivy ScreenManager breaks BoxLayout

Question

I am new to Kivy but was able to create a one screen app. It all looked good until I need to add more screens. The one page app displayed correctly but when I added screens the BoxLayouts no longer worked as I expected. The widgets all rendered on top of each other at the botttom of the screen. My question is what am I doing wrong in implementing ScreenManager?

The one screen app looked like this:

I decided to make it multi screen but when I added the ScreenManager logic, the resulting screen looked like this:

I took the information from this tutorial

My main.py file looks like this:

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.uix.popup import Popup
from kivy.properties import ObjectProperty
from kivy.uix.screenmanager import ScreenManager, Screen, FadeTransition

class Pay_screen(Screen):
    pass

class Survey_screen(Screen):
    pass

class Finish_screen(Screen):
    pass

class Sm(ScreenManager):
    pass

class Next_root(BoxLayout):
    pass

class Nq_disagree_label(Label):
    pass

class Nq_question_label(Label):
    pass

class Nq_button(Button):
    pass

class survey_form(BoxLayout):
    #dept_button = ObjectProperty()

    def send_survey(self):
        mypopup = MyPopup()
        mypopup.show_popup('Survey', 'Survey sent!', 'OK!')

    def add_comment(self):
        mypopup = MyPopup()
        mypopup.show_popup('Comment', 'This is where a comment is added.', 'OK!')

    def close_app(self):
        App.get_running_app().stop()

class MyPopup(Popup):
    def show_popup(self, title_text, label_text, button_text):
        mytext= label_text
        content = BoxLayout(orientation="vertical")
        content.add_widget(Label(text=mytext, font_size=20, text_size=(300, None)))
        mybutton = Button(text="Ok!", size_hint=(1,.20), font_size=20)
        content.add_widget(mybutton)
        mypopup = Popup(content = content,              
                        title = title_text,     
                        auto_dismiss = False,         
                        size_hint = (.5, .5)) #,         
                        #font_size = 20)
        mybutton.bind(on_press=mypopup.dismiss)  
        mypopup.open()  

class nextqualApp(App):
    icon = 'nextqual.png'
    title = 'Pay / survey / join'


if __name__ == '__main__':

    nextqualApp().run()

My KV file with the screen manager looks like this:

#: import FadeTransition kivy.uix.screenmanager.FadeTransition

Sm:
    transition: FadeTransition()
    Survey_screen:
    Pay_screen:


<nq_button@Button>:
    halign: "center"
    text: "Add\ncomment"
    size_hint_x: 10

<Nq_disagree_label@Label>:
    halign: "left"
    size_hint_x: 10
    text:"Strongly\ndisagree"

<Nq_agree_label@Label>:
    halign: "left"
    size_hint_x: 10
    text:"Strongly\nagree"

<Nq_question_label@Label>:
    halign: "left"
    font_size: "24"
    size_hint_x: 25

<Pay_screen>:
    name: 'pay'
    #orientation: "vertical"
    #padding: 6
    #font_size: 24
    BoxLayout:
        height: "40dp"
        Button: 
            text: "Pay your bill"
            on_release: app.root.current = 'pay'
        Button:
            text: "Tell us how we did"
            on_release: app.root.current = 'survey'
        Button:
            text: "I'm finished"
    BoxLayout:
        Button:
            text: "Pay your bill"
            on_release: app.root.current = 'survey'

<Survey_screen>:
    name: "survey"
    #orientation: "vertical"
    #padding: 6
    #font_size: "24"

    BoxLayout:
        height: "40dp"
        Button: 
            text: "Pay your bill"
            on_release: app.root.current = 'pay'

        Button:
            text: "Tell us how we did"
            on_release: app.root.current = 'survey'
        Button:
            text: "I'm finished"

    BoxLayout:
        halign: "center"
        Image:
            source: "logo.jpg"  
            #size: self.size

    BoxLayout:
        height: "80dp"
        size_hint_y: None
        Label:
            bold: True
            #color: 10,10,10,10
            #halign: "center"
            #text_size: self.size
            markup: True
            text: "[color=f9f752]Description[/color]"
            multiline: True
            #size_hint_x: 25
            text_size: self.size
            halign: 'center'
            valign: 'middle'


    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_question_label:
            text: "The service I recieved from start to finish was excellent."

    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_disagree_label:
        Slider:
            size_hint_x: 80
            value: 50
        Nq_agree_label:
        nq_button:
            on_press: root.add_comment()

    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_question_label:
            text: "I waited an appropriate amount of time for my food and drink."
    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_disagree_label:
        Slider:
            size_hint_x: 80
            value: 50
        Nq_agree_label:
        nq_button:
            on_press: root.add_comment()


    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_question_label:
            text: "The quality and presentation of the food was outstanding."
    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_disagree_label:
        Slider:
            size_hint_x: 80
            value: 50
        Nq_agree_label:
        nq_button:
            on_press: root.add_comment()


    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_question_label:
            text: "The prices provide good value for money."
    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_disagree_label:
        Slider:
            size_hint_x: 80
            value: 50
        Nq_agree_label:
        nq_button:
            on_press: root.add_comment()

    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_question_label:
            text: "The atmosphere was relaxing and enjoyable."
    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_disagree_label:
        Slider:
            size_hint_x: 80
            value: 50
        Nq_agree_label:
        nq_button:
            on_press: root.add_comment()

    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_question_label:
            text: "The overall cleanliness was very acceptable."
    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_disagree_label:
        Slider:
            size_hint_x: 80
            value: 50
        Nq_agree_label:
        nq_button:
            on_press: root.add_comment()

    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_question_label:
            text: "I will recommend Synterest to my friends and family."
    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_disagree_label:
        Slider:
            size_hint_x: 80
            value: 50
        Nq_agree_label:
        nq_button:
            on_press: root.add_comment()


    BoxLayout:
        height: "100dp"
        size_hint_y: None

        Label:
            size_hint_x: 40
        Button:
            size_hint_x: 20
            text:"Send survey"
            halign: "center"
            on_press: root.send_survey()
        Label:
            size_hint_x: 40

The KV file without the screen manager looks like this:

nq_button@Button>:
    halign: "center"
    text: "Add\ncomment"
    size_hint_x: 10

<Nq_disagree_label@Label>:
    halign: "left"
    size_hint_x: 10
    text:"Strongly\ndisagree"

<Nq_agree_label@Label>:
    halign: "left"
    size_hint_x: 10
    text:"Strongly\nagree"

<Nq_question_label@Label>:
    halign: "left"
    font_size: "24"
    size_hint_x: 25

survey_form:

<survey_form>:
    name: "survey"
    orientation: "vertical"
    padding: 6
    font_size: "24"

    BoxLayout:
        height: "40dp"
        Button: 
            text: "Pay your bill"
            on_release: app.root.current = 'pay'

        Button:
            text: "Tell us how we did"
            on_release: app.root.current = 'survey'
        Button:
            text: "I'm finished"

    BoxLayout:
        halign: "center"
        Image:
            source: "logo.jpg"  
            #size: self.size

    BoxLayout:
        height: "80dp"
        size_hint_y: None
        Label:
            bold: True
            #color: 10,10,10,10
            #halign: "center"
            #text_size: self.size
            markup: True
            text: "[color=f9f752]At Synterest, we care deeply about your impressions of our food, service and atmosphere. We would be very grateful if you took a few moments to give us your feedback on how you enjoyed your meal.[/color]"
            multiline: True
            #size_hint_x: 25
            text_size: self.size
            halign: 'center'
            valign: 'middle'


    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_question_label:
            text: "The service I recieved from start to finish was excellent."

    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_disagree_label:
        Slider:
            size_hint_x: 80
            value: 50
        Nq_agree_label:
        nq_button:
            on_press: root.add_comment()

    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_question_label:
            text: "I waited an appropriate amount of time for my food and drink."
    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_disagree_label:
        Slider:
            size_hint_x: 80
            value: 50
        Nq_agree_label:
        nq_button:
            on_press: root.add_comment()


    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_question_label:
            text: "The quality and presentation of the food was outstanding."
    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_disagree_label:
        Slider:
            size_hint_x: 80
            value: 50
        Nq_agree_label:
        nq_button:
            on_press: root.add_comment()


    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_question_label:
            text: "The prices provide good value for money."
    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_disagree_label:
        Slider:
            size_hint_x: 80
            value: 50
        Nq_agree_label:
        nq_button:
            on_press: root.add_comment()

    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_question_label:
            text: "The atmosphere was relaxing and enjoyable."
    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_disagree_label:
        Slider:
            size_hint_x: 80
            value: 50
        Nq_agree_label:
        nq_button:
            on_press: root.add_comment()

    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_question_label:
            text: "The overall cleanliness was very acceptable."
    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_disagree_label:
        Slider:
            size_hint_x: 80
            value: 50
        Nq_agree_label:
        nq_button:
            on_press: root.add_comment()

    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_question_label:
            text: "I will recommend Synterest to my friends and family."
    BoxLayout:
        height: "40dp"
        size_hint_y: None
        Nq_disagree_label:
        Slider:
            size_hint_x: 80
            value: 50
        Nq_agree_label:
        nq_button:
            on_press: root.add_comment()


    BoxLayout:
        height: "100dp"
        size_hint_y: None

        Label:
            size_hint_x: 40
        Button:
            size_hint_x: 20
            text:"Send survey"
            halign: "center"
            on_press: root.send_survey()
        Label:
            size_hint_x: 40

Answer 1

After sweating over this for a long time, it sometimes the answer becomes obvious.

Survey_screen and Pay_screen are inheriting from Screen in the Main.py file. When I made the original one page app, the root widget inherited from BoxLayout.

I added a BoxLayout to each screen in the .kv file and that put everything in the right place.

For example,

<Survey_screen>:
    name: "survey"
    BoxLayout:
        orientation: "vertical"
        padding: 6
        BoxLayout:
            height: "40dp"
            size_hint_y: None

            Button: 

etc

Answer 2

I have managed to get screen manager works, and share widget properties across all screens.

class XDispLayout(Screen):

    intr_str01 = str('This application advises --')


    def start01_value(self):

      xclr=[random.random(), random.random(), random.random()]+[1]

      self.ids.x1.color=xclr
      self.ids.set_ipbtn.color=xclr
      app = App.get_running_app()
      print ('text imported is '+ str(app.INT_TEXT))
      app.INT_TEXT = app.INT_TEXT + 1

      app.COLOR_STX=xclr
      print ('screen-Main app.COLOR_STX -is >'+ str(app.COLOR_STX))
      self.ids.ip_add01.color=app.COLOR_STX
      #below is an example of how you update on another screen, Year05 screen for eg.
      app.root.pyear05.ids.sfayear05.color =app.COLOR_STX

      return()

class Year01(Screen):
    def set_chk_same_cshint_y1(self,guess, *arg):
        if guess.active:
           temp_int=float(self.ids.cshint_y1m1.value)
           print ('tttt-is >'+ str(temp_int))
           app = App.get_running_app()
           self.ids.sfayear06.color=app.COLOR_STX
           print ('app.root.pyear01.ids.int_ylm2.value -is >'+ str(app.root.pyear01.ids.int_ylm1.value))
           #since the above is actually the same object, you can use 
           #different ways....
           #self.ids.int_ylm1.value==temp_int
           #app.root.pyear01.ids.int_ylm1.value=temp_int



class CManager(ScreenManager):
      #below is important when you want to change something on another screen
      pmain = ObjectProperty(None)
      pyear01 = ObjectProperty(None)
      pyear02 = ObjectProperty(None)
      pyear03 = ObjectProperty(None)
      pyear04 = ObjectProperty(None)
      pyear05 = ObjectProperty(None)
      pyear06 = ObjectProperty(None)
      #self.transition=NoTransition()
      pass


class FinDispApp(App):

  # the below values can be accessed using app.INT_TEXT, app.COLOR_STX
  INT_TEXT = 55
  COLOR_STX = [1,0,0,1]
  CASHOUT_00=0

  #.....
  #.......

   def build(self):
      sm = CManager(transition=NoTransition())
      sm.current='main'
      return sm


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



<CManager>:
    #this is the root widget
    id:screen_manager01
    #below maps python object property to kivy Ids
    pmain : idmain
    pyear01 : idyear01
    pyear02 : idyear02
    pyear03 : idyear03
    pyear04 : idyear04
    pyear05 : idyear05
    pyear06 : idyear06

    XDispLayout:
        id:idmain
        name:'main'
        manager:screen_manager01
    Year01:
        id:idyear01
        name:'year01'
        manager:screen_manager01
    Year02:
        id:idyear02
        name:'year02'
        manager:screen_manager01
    Year03:
        id:idyear03 
        name:'year03'
        manager:screen_manager01
    Year04:
        id:idyear04
        name:'year04'
        manager:screen_manager01
    Year05:
        id:idyear05
        name:'year05'
        manager:screen_manager01
    Year06:
        id:idyear06
        name:'year06'
        manager:screen_manager01

<XDispLayout>:
    name: 'main'
    orientation: "horizontal"
    BoxLayout:
        orientation:'vertical'


        BoxLayout:
            orientation:'horizontal'
            size_hint_x :1
            size_hint_y :0.08
            Button:
                id: btn_yr06
                text:'go to year06'
                o#pacity : 0
                #disabled : True
                on_press:
                    #root.setvalue_00()
                on_release:
                    root.manager.current = 'year06'
                    root.manager.transition.direction = 'right'               
            Button:
                id: btn_yr01
                text:'go to year01'
                opacity : 0
                disabled : True
                on_press:
                    #root.enable_y06()
                    #root.setvalue_00()
                on_release:
                    root.manager.current = 'year01'
                    root.manager.transition.direction = 'left'


<Year01>:
    name: 'year01'
    BoxLayout:
        orientation:'vertical'
        Label:
            id:sfayear01
            text: "Year01"


<Year02>:
    name: 'year02'
    BoxLayout:
        orientation:'vertical'
        Label:
            id:sfayear02
            text: "Year02"
            halign : 'right'
            valign : 'middle'


<Year06>:
    name: 'year06'
    BoxLayout:
        orientation:'vertical'
        Label:
            id:sfayear06
            text: "blah blah blas"
            font_size: 20

        BoxLayout:
            orientation:'horizontal'
        BoxLayout:
            orientation:'horizontal'
            size_hint_x :1
            size_hint_y :0.4
            Button:
                text:'go to year05'
                on_press:
                    root.manager.current = 'year05'
                    root.manager.transition.direction = 'right'
            Button:
                text:'go to main'
                on_press:
                    root.manager.current = 'main'
                    root.manager.transition.direction = 'left'