app.message listeners do not catch messages with subtype: thread_broadcast #488

When an end-user posts a new message in a thread with "Also send to #channel" checked, the message's event payload has "subtype": "thread_broadcast". In this scenario, bolt-python's app.message listeners are not called even if the text matches with the listener constraints.

This issue was originally reported by @kanny -- thank you!
https://community.slack.com/archives/CHY642221/p1633662076075800

Notes on other Bolt frameworks:

• bolt-js invokes a message listener for the pattern without any issues
• bolt-java's message listener is different from other bolt frameworks (to support the language's static types, a single message listener does not handle multiple subtypes; the recommended way is to use multiple app.event listeners)

Reproducible in:

Any

The slack_bolt version

~ v1.9.2

Python runtime version

Any

OS info

Any

Steps to reproduce:

from slack_bolt import App

app = App()

@app.message("Hi there!")
def handle_messages(event, logger):
    logger.info(event)

1. Start a new thread
2. Post a reply including "Hi there!" in the thread with "Also send to #channel" checked

Expected result:

The logger prints the event payload

Actual result:

The event is not handled

Requirements

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