[BUG] A progress callable of a bg callback sometimes doesn't trigger. about dash HOT 2 OPEN

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.

If you have a sleep for 1 second, before setting "running" it is never going to appear right with the default settings.
The background callbacks operates a chains of requests, it wait for interval argument which by default is 1000 ms. You can set the interval to a lower amount to ensure faster retrieval of progress values.

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The documentation is extremely unclear of the inner workings. The https://dash.plotly.com/background-callbacks sections does not mention the behaviour you mentioned at all and does not mention any limitations for the bg callback progress callable.
API reference just says "interval - Time to wait between the long callback update requests.", which explains very little.

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