[BUG] Setting children to None leaves removed components registered: their callbacks keep firing and the subtree can never be mounted again

[vdidon]

Summary: if a callback removes a subtree by returning children=None, the removed components stay registered: callbacks targeting them keep firing, and the subtree can never be rendered again. Returning [] works correctly.

Describe your context

dash                 4.4.1  (same on 4.5.0rc0, 4.0.0, 3.3.0, 2.18.2)
plotly               6.6.0

• Python 3.10, Windows 11, Microsoft Edge 154 (Chromium)
• Same with debug=True and debug=False, no error in the console or the dev tools

Describe the bug

from dash import Dash, Input, Output, ctx, dcc, html

app = Dash(__name__, suppress_callback_exceptions=True)

app.layout = html.Div([
    dcc.Interval(id="tick", interval=1000),
    html.Button("Mount", id="mount"),
    html.Button("Unmount", id="unmount"),
    html.Div(id="container"),
])


@app.callback(
    Output("container", "children"),
    Input("mount", "n_clicks"),
    Input("unmount", "n_clicks"),
    prevent_initial_call=True,
)
def toggle(_mount, _unmount):
    if ctx.triggered_id == "mount":
        return html.Div(id="view", children=html.Div(id="view-tick"))
    return None  # returning [] instead works as expected


@app.callback(Output("view-tick", "children"), Input("tick", "n_intervals"))
def update_view(n):
    print(f"update_view called, n_intervals={n}", flush=True)
    return f"tick {n}"


if __name__ == "__main__":
    app.run(debug=False)

1. Click Mount: tick N is displayed, update_view runs every second. Correct.
2. Click Unmount: the text disappears, but the browser keeps sending update_view requests every second (visible in the server log).
3. Click Mount again: the toggle request returns the subtree (HTTP 200), but #view never appears in the DOM, until the page is reloaded.

With return [] in toggle, step 2 sends no request and step 3 displays the view again.

Expected behavior

None and [] should both remove the subtree: no more callbacks for the removed components, and the subtree can be mounted again.