Consider making `page_fillable()` the default for `shiny.express`

[cpsievert]

This way, top-level fill items like plots, cards, or widgets, will fill the page how you might expect them to, for example:

from shiny import render, ui
from shiny.express import input, layout

# This could go away
# layout.set_page(layout.page_fillable())

@render.plot
def hist():
    import matplotlib.pyplot as plt
    import seaborn as sns
    df = sns.load_dataset("penguins")
    fig, ax = plt.subplots()
    sns.histplot(df, x="bill_length_mm", hue="species", ax=ax)
    return fig

Another benefit of doing this is that we might not have to change the page layout for a top-level with layout.sidebar():

BTW, @wch, the last line in that screenshot should be layout.set_page(layout.page_fixed())

[gshotwell]

I don't like having to call both layout.sidebar() and sidebar to get a sidebar layout. The most familiar interface for people is probably the Streamlit one which uses the context manager to just set the sidebar elements:

st.subheader("header 1")

with st.sidebar:
    sample_ui = st.number_input(
        "sample", 0.0, 1.0, value=0.1, step=0.01, on_change=increment_counter
    )
    log = st.checkbox("Log Scale")

st.subheader("header 2")

So I think the current implementation is probably closer to what people are expecting:

ui.h2("header 1")

with layout.sidebar():
    ui.input_numeric("n", "N")
    ui.input_checkbox("check", "Checkbox")

ui.h2("header 2")