dcc.Loading is not accessible by DOM in Dash 2.18.2

[Kissabi]

Description

The dcc.Loading component's children and elements within custom_spinner are not accessible via document.getElementById in clientside callbacks when using Dash 2.18.2. This prevents direct DOM manipulation within these elements.

Steps to Reproduce

The following minimal example demonstrates the issue:

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

app = Dash(prevent_initial_callbacks=True)

app.layout = html.Div([
    html.Button("Button 1", id="btn1"),
    html.Button("Button 2", id="btn2"),
    html.Button("Button 3", id="btn3"),
    dcc.Loading(html.P(id="logA"), overlay_style={"visibility":"visible", "opacity": 0.5, "backgroundColor": "#000"}, custom_spinner=html.Div(id="logB"), id="logC")
])

# Works (accessing direct child of layout)
app.clientside_callback(
    """
    async function(){
        const content = document.getElementById("logA");

        if (!content) {
            console.error("content element not found!");
            return;
        }
		
        await new Promise(r => setTimeout(r, 10000)); 
        return "Test";
    }
    """,
    Output("logA", "children"),
    Input("btn1", "n_clicks"),
    prevent_initial_call=True
)

# Does not work (accessing custom_spinner element)
app.clientside_callback(
    """
    function(){
        const content = document.getElementById("logB");

        if (!content) {
            console.error("content element not found!");
            return;
        }
        
        return "loading test...";
    }
    """,
    Output("logB", "children"),
    Input("btn1", "n_clicks"),
    prevent_initial_call=True
)

# Does not work (accessing the Loading component itself)
app.clientside_callback(
    """
    function(){
        const content = document.getElementById("logC");

        if (!content) {
            console.error("content element not found!");
            return;
        }
        
        return "loading test...";
    }
    """,
    Output("logC", "children"),
    Input("btn1", "n_clicks"),
    prevent_initial_call=True
)

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

Expected Behavior

All elements with assigned IDs (logA, logB, and logC) should be accessible via document.getElementById within clientside callbacks.

Actual Behavior

Only logA (the direct child of the layout, outside custom_spinner and not the dcc.Loading component itself) is accessible. logB (inside custom_spinner) and logC (the dcc.Loading component) are not.

[AnnMarieW]

I don't think this is a bug. The loading spinner is available in the dom only when the component is loading.

If you change the second callback to add a delay, to allow some time for the spinner to render it works fine. The third callback should be deleted because it replaces the component (logA) that is being updated and makes no sense to do that.

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

app = Dash(prevent_initial_callbacks=True)

app.layout = html.Div([
    html.Button("Button 1", id="btn1"),
    html.Button("Button 2", id="btn2"),
    html.Button("Button 3", id="btn3"),
    dcc.Loading(html.P(id="logA"), overlay_style={"visibility": "visible", "opacity": 0.5, "backgroundColor": "#000"},
                custom_spinner=html.Div(id="logB"), id="logC")
])

# Works (accessing direct child of layout)
app.clientside_callback(
    """
    async function(){
        const content = document.getElementById("logA");

        if (!content) {
            console.error("content element not found!");
            return;
        }

        await new Promise(r => setTimeout(r, 10000)); 
        return "Test";
    }
    """,
    Output("logA", "children"),
    Input("btn1", "n_clicks"),
    prevent_initial_call=True
)

app.clientside_callback(
    """
    function(n_clicks) {
        if (!n_clicks) {
            return null;
        }

        // Poll until the element is available
        return new Promise((resolve) => {
            const interval = setInterval(() => {
                const content = document.getElementById("logB");
                if (content) {
                    clearInterval(interval);
                    resolve("loading test...");
                }
            }, 100); // Check every 100ms
        });
    }
    """,
    Output("logB", "children"),
    Input("btn1", "n_clicks"),
    prevent_initial_call=True
)

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