fix: flatten outputs before building loadingOutputs

[i-murray]

Summary

Fix dcc.Loading spinner not triggering when a callback Output uses the ALL wildcard.

Problem

In dash-renderer/src/actions/callbacks.ts, loadingOutputs was built with outputs.map(...). When the output spec contains ALL, unwrapIfNotMulti returns resolved outputs as a nested array. Iterating with .map() over this nested structure meant out.id was undefined, so getPath returned undefined and the loading reducer never matched the component path.

Fix

Changed outputs.map(...) to flatten(outputs).map(...) on L794, consistent with how outputs are already flattened on L829 and L912 in the same function.

Fixes #3619

[AnnMarieW]

Thanks for the fix @i-murray

I tried your sample app after the fix and verified that it works. Here is a simplified version that can be added as a test in this file: https://github.com/plotly/dash/blob/dev/components/dash-core-components/tests/integration/loading/test_loading_component.py

def test_ldcp019_loading_component_pattern_matching(dash_dcc):
    lock = Lock()

    app = Dash(__name__)

    app.layout = html.Div(
        [dcc.Loading([html.Div(id={"type": "div-1", "index": 1, "name": "test"}, className="div-1")], className="loading")],
        id={"type": "root", "index": 1, "name": "test"},
        className="root"
    )

    @app.callback(Output({"type": "div-1", "index": ALL, "name": MATCH}, "children"), Input({"type": "root", "index": ALL, "name": MATCH}, "n_clicks"))
    def updateDiv(n_clicks):
        if n_clicks ==  [1]:
            time.sleep(.1)
            return ["changed"]
        return ["content"]

    with lock:
        dash_dcc.start_server(app)
        dash_dcc.wait_for_text_to_equal(".div-1", "content")

        dash_dcc.find_element(".root").click()

        dash_dcc.find_element(".loading .dash-spinner")
        # mounted but hidden, so looks like no text
        dash_dcc.wait_for_text_to_equal(".div-1", "")

    dash_dcc.wait_for_text_to_equal(".div-1", "changed")

    assert dash_dcc.get_logs() == []

[AnnMarieW]

@T4rk1n This is ready for review - I ran the new test locally, but I don't think I can add it to the PR since I'm not a maintainer.

[i-murray]

@AnnMarieW, thank you for writing the test for this fix. I've added it.

[T4rk1n]

💃 Looks good, just need to run npm run format to pass the linting.

[ndrezn]

Thanks for the contribution @i-murray !