Trouble with sizing altair/vega charts

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Even when setting both altair's .properties() and the underlying vega ipywidget's sizing API, there is no effect on the size...

from shiny import App, ui
from shinywidgets import output_widget, render_widget, as_widget
import altair as alt
from vega_datasets import data

app_ui = ui.page_fluid(
    output_widget("plot")
)

def server(input, output, session):

    @output(id="plot")
    @render_widget
    def _():
        cars = data.cars()
        fig = alt.Chart(cars).mark_point().encode(
            x='Horsepower',
            y='Miles_per_Gallon',
            color='Origin',
        )
        fig.properties(height=600)
        w = as_widget(fig)
        w.layout.height = "600px"
        return w

app = App(app_ui, server)