Layout as list of components does not work with Dash Pages page_container

[antonymilne]

Describe your context

dash                       2.17.1
dash_ag_grid               31.2.0
dash-bootstrap-components  1.6.0
dash-core-components       2.0.0
dash-html-components       2.0.0
dash-mantine-components    0.12.1
dash-table                 5.0.0
dash-testing-stub          0.0.2

Describe the bug

#2795 enabled you to pass a list of components to app.layout. However, this does not work with page_container.

import dash

from dash import Dash, html, page_container

app = Dash(__name__, use_pages=True, pages_folder="")
dash.register_page("/", layout=html.H1("Hello"))
app.layout = [page_container, html.H2("Stuff")]
# These work ok:
# app.layout = page_container
# app.layout = html.Div([page_container, html.H2("Stuff")])

app.run()

The first request will give the following exception:

Exception: `dash.page_container` not found in the layout

Subsequent page refreshes are ok because this check only occurs on the first request.

The problem is that the following check is failing:

dash/dash/dash.py

Line 2257 in bbd013c

if _ID_CONTENT not in self.validation_layout:

The reason for that is as follows:

assert "a" in html.Div(html.Div(id="a")) # passes
assert "a" in html.Div([html.Div(id="a")]) #passes
assert "a" in html.Div([[html.Div(id="a")]]) # FAILS - this is the relevant case here

As with #2905 probably the easiest solution is to wrap the layout in html.Div in this case so that the check passes:

assert "a" in html.Div([html.Div([html.Div(id="a")])]) # passes

Note that while #2905 is very similar it looks like the proposed fix #2915 will not solve this case. Given these two very similar issues, maybe there's a better elsewhere that would fix both (and any other similar undiscovered issues) simultaneously?

[kenshima4]

Hi dash team, I've opened a pull request for this. Can someone take a look and let me know if anything needs to be changed please? #3108