Considering using DataWriter on the StreamSocket OutputStream instead of the stream directly for QBluetoothSocket as OutputStream will indicate that it has written the data right away although this is not necessarily the case. By using DataWriter it is possible to follow the progress of the writing so we can emit bytesWritten() when they are actually written.

For reference:
https://github.com/microsoft/Windows-universal-samples/blob/master/Samples/BluetoothRfcommChat/cs/Scenario1_ChatClient.xaml.cs#L315 - shows the StreamSocket using a DataWriter