`page_fillable()`'s `fillable_mobile=False` no longer works as intended

[cpsievert]

For example, the plot here always fills the page (even in a small width window)

from shiny import App, render, ui

app_ui = ui.page_fillable(
    ui.output_plot("plt")
)

def server(input, output, session):

    @render.plot()
    def plt():
        import matplotlib.pyplot as plt
        import numpy as np
        x = np.linspace(0, 2 * np.pi, 100)
        y = np.sin(x * 50)
        fig, ax = plt.subplots()
        ax.plot(x, y)
        return fig

app = App(app_ui, server)

Looks like this problem was recently introduced by #819