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The serial driver drops RFCOMM hosts that don't send a Pybricks message within 5 seconds (e.g. the macOS Bluetooth panel, which opens the port when connecting). On NXT, this disconnect also stopped the Bluetooth process, which was only meant to happen on shutdown. Nothing could connect after that until the hub was restarted. Drop just the connection and go back to listening, as on BTstack. A shutdown still waits for the connection to close.
If the host closes the connection while the hub is sending (e.g. stdout at the end of a program), the BC4 can stop taking data without dropping the stream. The transmission then never finishes, which stalls stdout on all transports and leaves the BC4 unable to accept a new connection. Give up after 1 second, as the USB driver does after 500 ms, and drop the link. The BC4 does not take commands in this state, so go straight to the reset that restarts it instead of closing the connection first.
The BC4 was reset after every connection. This drops the radio link without telling the host, which then fails to connect again for a while (e.g. macOS keeps trying the old link). Each failed attempt ended in another reset, so reconnecting could fail indefinitely. Once the BC4 reports the connection closed, wait for the next one without a reset. Only reset it if it stops responding.
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Fixes Bluetooth on NXT getting stuck after a disconnect:
Known limitation: disconnecting mid-transfer still needs a BC4 reset, after which macOS fails the first reconnect attempt. Unsubscribing before closing avoids this.
Tested on hardware with a macOS host.