Kivy: layout size from child

layoutsize.py

import kivy
kivy.require('1.9.0')

from kivy.app import App
from kivy.lang import Builder

root = Builder.load_string('''
<Message@BoxLayout>:
    name: ''
    message: ''
    size_hint_y: None
    height: max(lbl_name.height, lbl_message.height)
    Label:
        id: lbl_name
        pos_hint: {'top': 1}
        size_hint_y: None
        size_hint_x: 0.5
        text: root.name
        height: self.texture_size[1]
    Label:
        id: lbl_message
        pos_hint: {'top': 1}
        size_hint_y: None
        text_size: self.width, None
        text: root.message
        height: self.texture_size[1]

ScrollView:
    GridLayout:
        cols: 1
        size_hint_y: None
        height: self.minimum_height

        Message:
            name: 'user 1'
            message: 'hello there'
        Message:
            name: 'user 2'
            message: 'why hello!'
        Message:
            name: 'user 1'
            message: 'i want to test a long message, so that it will wrap to multiple lines and extend the size of the box'
        Message:
            name: 'user 2'
            message: ''
        Message:
            name: 'user 2'
            message: 'that was a blank message'
        Message:
            name: 'user 1'
            message: 'ok!'
''')

class TestApp(App):
    def build(self):
        return root

if __name__ == '__main__':
    TestApp().run()