Silently setting use_async to True when asgiref is available is non-obvious

[ckrivik]

A few days after upgrading to Dash 4.4.0, I was having an intermittent issue due to a race condition caused by use_async turning on silently. It was an easy fix to explicitly set use_async=False, but it was non-obvious at first that this was the issue.

I'm not sure that I'd call this a "bug" since this does seem like it's intended behaviour, but it was confusing that it happened silently, so I'd like to propose some ideas that might help make this more explicit:

• The default value for this could be set to False so that use_async isn't enabled without a clear signal of user intent. Users could still explicitly provide None if they really want use_async to be determined dynamically based on availability of asgiref.
• Dash constructor could log a warning when it decides to automatically enable async, with a hint to set use_async=False if this was unintentional/undesired.
• Documentation could be added/updated to explain this functionality, maybe in the docstrings for the Dash() constructor, the API Reference Page, and/or a "Migrating to Dash 4" page?

Hope this helps!

[robertclaus]

@ckrivik thanks for reporting that you ran into this! Could you add more details on the bug this caused for you? We didn't expect this to cause issues for anyone so it would be helpful to understand how exactly it affected you so we can fix it in the best way.