progress callable of a bg callback sometimes doesn't trigger

python 3.10.13
dash 2.16.0

• if frontend related, tell us your Browser, Version and OS

  • OS: macos 14.4
  • Browser firefox
  • Version 124.0.2

Describe the bug

A function callable of a callback with a progress param doesn't seem to get hooked immediately. And calls to this callable seem to be ignored sometimes - especially fraction of a seconds after the callback itself has been triggered.

Expected behavior

MVE:

from dash import html, Input, Output, Dash, callback, DiskcacheManager
import diskcache
import time

cache = diskcache.Cache("./cache")
background_callback_manager = DiskcacheManager(cache)

app = Dash(__name__, background_callback_manager=background_callback_manager)
app.layout = html.Div(
    [html.Div(id="result"), html.Div(id="progress"), html.Button("Run", id="run")]
)


@callback(
    [Output("result", "children")],
    [Input("run", "n_clicks")],
    prevent_initial_call=True,
    background=True,
    running=[(Output("run", "disabled"), True, False)],
    progress=[Output("progress", "children")],
)
def run(set_progress, n):
    # time.sleep(1)
    set_progress("running")
    time.sleep(1)
    set_progress("still running")
    time.sleep(1)  # long running function
    set_progress("finished")
    return (f"Run clicked {n} times",)


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

Run this app and click the button.

Expected behaviour:

• it shows "running immediately after pressing the button. Then after a second, "still sunning" is shown and after yet another second "finished" is displayed and stays on the screen.

Observed behaviour:

• after clicking a button, nothing changes for a second, and then it shows "still running". The first call to set_progress at the beginning of the callback seems to be ignored.
• if you uncomment the time.sleep call, then the first call to set_progress seems to go through.

Run the provided MVE using Dash, DiskcacheManager, and the background callback with progress to reproduce the missed initial set_progress call. Start by tracing the background callback and progress handling entry points shown in the example. Done means the initial, intermediate, and final progress values appear reliably, including when no initial sleep is used.