main.py

from kivy.config import Config
Config.set('kivy', 'log_level', 'debug')

from kivy.app import App
from kivy.lang import Builder
from kivy.factory import Factory
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.gridlayout import GridLayout
from kivy.uix.modalview import ModalView
from kivy.properties import ObjectProperty
from kivy.metrics import *

kv = """
<CustomContent>:
    button_1: button_1
    size_hint: None, None
    width: dp( 275 )
    height: dp( 25 )
    orientation: 'horizontal'

    Button:
        id: button_1
        size_hint: 0.9, 1.0
        text_size: self.size
        halign: 'left'
        valign: 'middle'
        markup: True
        text: root.text

        canvas:
            Color:
                rgba: root.colour
            Rectangle:
                pos: self.pos[ 0 ] + 1, self.pos[ 1 ]
                size: 10, self.height

    Button:
        size_hint: None, 1.0
        width: dp( 30 )
        border: 0, 0, 0, 0
        text: 'B'

"""


class CustomContent(BoxLayout):
	text = ObjectProperty( )
	colour = ObjectProperty( )
	pass

class TestApp(App):
    def build(self):
        button = CustomContent( text = "[b]  Hello[/b]", colour = ( 1.0, 0, 0, 1 ) )

        grid = GridLayout( size_hint_y = None,
                           cols = 1,
                           spacing = ( 20, 0 ),
                           padding = ( 0, 0, 0, 0 ) )
        grid.add_widget( button )

        popup = ModalView( size_hint = ( None, None ),
                           width = dp( 275 ),
                           height = dp( 0 ),
                           background_color = ( 0, 0, 0, 0 ) )
        popup.add_widget( grid )
        popup.height = grid.minimum_height
        popup.open( )

        print( "popup.size: {}".format( popup.size ) )
        print( "grid.size:  {}".format( grid.size ) )
        print( "grid.miminum_height: {}".format( grid.minimum_height ) )
        print( "button.size: {}".format( button.size ) )
        print( "button.button_1.size: {}".format( button.button_1.size ) )

if __name__ == '__main__':
    Builder.load_string(kv)
    TestApp().run()