Limiting application width

I'm trying to specify a maximum width for my application's root container to avoid long lines in large terminals. I'm using an HSplit for the root container. Its width parameter does nothing unless I wrap it in a VSplit.

Is this a bug or am I misunderstanding how this should work?

Here's a simple demo:

from prompt_toolkit.application import Application
from prompt_toolkit.key_binding import KeyBindings
from prompt_toolkit.layout import Layout
from prompt_toolkit.layout.containers import HSplit, VSplit, Window
from prompt_toolkit.layout.controls import FormattedTextControl
from prompt_toolkit.styles import Style

style = Style([('text', 'bg:#a0a0a0 fg:#000000')])

root_container = HSplit(
    width=20,
    children=[
        Window(FormattedTextControl('Hello'),
               style='class:text',
               dont_extend_width=True),
    ],
)

# This extends root_container's width beyond 20
layout = Layout(
    root_container
)

# This limits root_container's width to 20
layout = Layout(
    VSplit([root_container])
)

kb = KeyBindings()

@kb.add('c-c')
def _(event):
    event.app.exit(1)

app = Application(layout=layout,
                  key_bindings=kb,
                  style=style)
app.run()

Run the provided Python demo first, then read the Layout, HSplit, VSplit, and Window entry points to trace how the root container receives its width. Done when it is clear whether this is intended and, if not, the demo's root HSplit respects the requested 20-column limit.