ConditionalContainer set a space on top

Hi,

I'm using conditional containers to select which container is shown based on a state variable. Each hidden container is adding a space on top.

#!/usr/bin/env python
"""
A simple example of a few buttons and click handlers.
"""
from prompt_toolkit.application import Application
from prompt_toolkit.application.current import get_app
from prompt_toolkit.key_binding import KeyBindings
from prompt_toolkit.key_binding.bindings.focus import focus_next, focus_previous
from prompt_toolkit.layout import HSplit, Layout, VSplit, HorizontalAlign, VerticalAlign
from prompt_toolkit.styles import Style
from prompt_toolkit.filters import Condition
from prompt_toolkit.completion import WordCompleter
from prompt_toolkit.widgets import Box, Button, Frame, Label, TextArea, Dialog
from prompt_toolkit.layout.containers import (
    AnyContainer,
    ConditionalContainer,
    Container,
    DynamicContainer,
    Float,
    FloatContainer,
    HSplit,
    VSplit,
    Window,
    WindowAlign,
)
from asyncio import Future, ensure_future
from prompt_toolkit.layout.menus import CompletionsMenu
from prompt_toolkit.layout.dimension import D

state = "main"

def clicked_main():
    global state
    state = "main"
    layout.focus((main_menu_container.get_children()[0]))

def clicked_item1():
    global state
    state = "item1"
    layout.focus((item1_menu_container.get_children()[0]))

def clicked_item2():
    global state
    state = "item2"
    layout.focus((item2_menu_container.get_children()[0]))

def clicked_item2_item21():
    global state
    state = "item21"
    layout.focus((item21_menu_container.get_children()[0]))

def clicked_exit():
    get_app().exit()

# All the widgets for the UI.
btn_item1 = Button("Item 1", handler=clicked_item1)
btn_item2 = Button("Item 2", handler=clicked_item2)
btn_item3 = Button("Item 3", handler=clicked_main)

btn_exit = Button("Exit", handler=clicked_exit)

btn_item1_item11 = Button("Item 1.1", handler=clicked_main)
btn_item1_item12 = Button("Item 1.2", handler=clicked_main)
btn_item1_item13 = Button("Item 1.3", handler=clicked_main)

btn_item2_item21 = Button("Item 2.1", handler=clicked_item2_item21)
btn_item2_item22 = Button("Item 2.2", handler=clicked_main)
btn_item2_item23 = Button("Item 2.3", handler=clicked_main)

btn_item2_item21_item211 = Button("Item 2.1.1", handler=clicked_main)
btn_item2_item21_item212 = Button("Item 2.1.2", handler=clicked_main)
btn_item2_item21_item213 = Button("Item 2.1.3", handler=clicked_main)

#MENU

main_menu_container = ConditionalContainer(
                        content=HSplit(
                            [
                                btn_item1, btn_item2, btn_item3
                            ], 
                            padding=1,
                        ),
                        filter=Condition(lambda: True if (state == "main") else False),
                    )

item1_menu_container = ConditionalContainer(
                        content=HSplit(
                            [
                                btn_item1_item11, btn_item1_item12, btn_item1_item13
                            ], 
                            padding=1,
                        ),
                        filter=Condition(lambda: True if (state == "item1") else False),
                    )

item2_menu_container = ConditionalContainer(
                        content=HSplit(
                            [
                                btn_item2_item21, btn_item2_item22, btn_item2_item23
                            ], 
                            padding=1,
                        ),
                        filter=Condition(lambda: True if (state == "item2") else False),
                    )

item21_menu_container = ConditionalContainer(
                        content=HSplit(
                            [
                                btn_item2_item21_item211, btn_item2_item21_item212, btn_item2_item21_item213
                            ], 
                            padding=1,
                        ),
                        filter=Condition(lambda: True if (state == "item21") else False),
                    )



quit_menu_container = VSplit(
                            [
                               btn_exit
                            ], 
                        )

root_container = FloatContainer(
    content=VSplit(
        [
            HSplit(
                [
                    main_menu_container,
                    item1_menu_container,
                    item2_menu_container,
                    item21_menu_container,
                    quit_menu_container,
                ],
                padding=1,
                align=HorizontalAlign.LEFT,
            ),
        ],
    ),
    floats=[
        Float(
            xcursor=True,
            ycursor=True,
            content=CompletionsMenu(max_height=16, scroll_offset=1),
        ),
    ],
)

layout = Layout(container=root_container)

# Key bindings.
kb = KeyBindings()
kb.add("down")(focus_next)
kb.add("up")(focus_previous)


# Styling.
style = Style(
    [
        ("button", "#00ff00"),
        ("button-arrow", "bg:#00ff00"),
        ("button focused", "bg:#00ff00"),
        ("text-area focused", "bg:#ff0000"),
        ("text-area", "bg:#ff0000"),
    ]
)


# Build a main application object.
application = Application(layout=layout, key_bindings=kb, style=None, full_screen=True)


def main():
    application.run()


if __name__ == "__main__":
    main()

How can I remove this space?

Run the provided reproduction through its main() entry point and inspect how ConditionalContainer instances are arranged inside HSplit with padding=1. Trace the layout behavior when the state changes, then verify that hidden containers no longer add vertical space while the visible menu and quit button retain their intended spacing.