First look at the table here:
If you look at the value of specialId and broadcastId, it works out to 0xC400 and 0xC403. This range is reserved for String messages for use by applications:
Message numbers in the fourth range (0xC000 through 0xFFFF) are defined at run time when an application calls the RegisterWindowMessage function to retrieve a message number for a string. All applications that register the same string can use the associated message number for exchanging messages. The actual message number, however, is not a constant and cannot be assumed to be the same between different sessions.
This means that applications will get these ideas by calling the RegisterWindowMessage function, and then they can send or broadcast those message accross applications. Excel and Word are doing this correctly, the call to RegisterWindowMessage() with whatever name they are using just happens to return 0xC403.
The solution will be to use the correct range for messages which are private to a given window class, which would be WM_USER through 0x7FFF: Integer messages for use by private window classes. I'm guessing the next range would also work.
Message numbers in the second range (WM_USER through 0x7FFF) can be defined and used by an application to send messages within a private window class. These values cannot be used to define messages that are meaningful throughout an application because some predefined window classes already define values in this range. For example, predefined control classes such as BUTTON, EDIT, LISTBOX, and COMBOBOX may use these values. Messages in this range should not be sent to other applications unless the applications have been designed to exchange messages and to attach the same meaning to the message numbers.
I think you can fix this issue without breaking backwards compatibility, as JUCE is sending WM_COPYDATA accross different apps, not broadcastId. Change the message value used internally for posting the message on the queue again, at:
In the other places, for backwards compatibility I think you'll have to keep the old value.