[BUG] Dash 1.11 initial callback edge case

[alexcjohnson]

There's another specific case when Dash 1.11 won't fire an initial callback:

• The output element was added to the layout after the initial render
• There's an input element higher up in the layout than the (first) output
• Another callback earlier in the chain called PreventUpdate

Minimal reproduction:

import dash

import dash_html_components as html
from dash.dependencies import Output, Input
from dash.exceptions import PreventUpdate

app = dash.Dash(__name__, suppress_callback_exceptions=True)

layout = html.Div([
    html.Div(42, id="above-in"),
    html.Div(id="above-dummy"),
    html.Hr(),
    html.Div(0, id='above-out'),
    html.Div(0, id='below-out'),
    html.Hr(),
    html.Div(id="below-dummy"),
    html.Div(44, id="below-in"),
])

app.layout = html.Div(id="content")


@app.callback(Output("content", "children"), [Input("content", "style")])
def content(_):
    return layout


# Create 4 callbacks - 2 for above, 2 for below.
for pos in ('above', 'below'):
    @app.callback(
        Output("{}-dummy".format(pos), "children"),
        [Input("{}-dummy".format(pos), "style")]
    )
    def dummy(_):
        raise PreventUpdate

    @app.callback(
        Output('{}-out'.format(pos), 'children'),
        [Input('{}-in'.format(pos), 'children')]
    )
    def on_data(data):
        return data


if __name__ == '__main__':
    app.run_server(debug=True)

Actually both callbacks fire, but the above-out callback return value is ignored (the "0" in the middle) while the below-out callback return is used correctly (the "44" in the middle):

Reported by @Marc-Andre-Rivet in plotly/dash-core-components#792 (comment)