You put the box inside another box...

The biggest conceptual idea in Toga is constructing a constraint-based layout. So, lets look at a more complex example, involving layouts, scrollers, and containers inside other containers.:

from __future__ import print_function, unicode_literals, absolute_import

import toga

def button_handler(widget):
    print("hello")

def action1():
    print("action 1")

def action2():
    print("action 2")

def action3():
    print("action 3")

def build(app):
    left_container = toga.Table(['Hello', 'World'])

    left_container.insert(None, 'root1', 'value1')
    left_container.insert(None, 'root2', 'value2')
    left_container.insert(None, 'root3', 'value3')
    left_container.insert(1, 'root4', 'value4')

    right_content = toga.Container()
    buttons = [
        toga.Button('Hello world %s' % b, on_press=button_handler)
        for b in range(0, 10)
    ]

    for i, button in enumerate(buttons):
        right_content.add(button)

        if i == 0:
            right_content.constrain(button.TOP == right_content.TOP + 50)
        else:
            right_content.constrain(button.TOP == buttons[i-1].BOTTOM + 50)
        right_content.constrain(button.LEADING == right_content.LEADING + 50)
        right_content.constrain(button.TRAILING + 50 == right_content.TRAILING)

    right_content.constrain(buttons[-1].BOTTOM + 50 < right_content.BOTTOM)

    right_container = toga.ScrollContainer()

    right_container.content = right_content

    split = toga.SplitContainer()

    split.content = [left_container, right_container]

    cmd1 = toga.Command(action1, 'Action 1', tooltip='Perform action 1', icon='icons/brutus.icns')
    cmd2 = toga.Command(action2, 'Action 2', tooltip='Perform action 2', icon=toga.TIBERIUS_ICON)
    cmd3 = toga.Command(action3, 'Action 3', tooltip='Perform action 3', icon='icons/brutus.icns')

    def action4():
        print ("CALLING ACtion 4")
        cmd3.enabled = not cmd3.enabled

    cmd4 = toga.Command(action4, 'Action 4', tooltip='Perform action 4', icon='icons/brutus.icns')

    app.main_window.toolbar = [cmd1, toga.SEPARATOR, cmd2, toga.SPACER, cmd3, toga.EXPANDING_SPACER, cmd4]

    return split

if __name__ == '__main__':
    app = toga.App('First App', 'org.pybee.helloworld', startup=build)

    app.main_loop()