Line 31 of BaseURLSourceImpl:
int port = request.getLocalPort();
Which calls same method in the underlying ServletRequest.
getLocalPort javadoc: "Returns the Internet Protocol (IP) port number of the interface on which the request was received."
getServerPort javadoc: "Returns the port number to which the request was sent. It is the value of the part after ":" in the <code>Host</code> header, if any, or the server port where the client connection was accepted on."
I think that the second is the one that should be used and since this port number is paired with the host returned from getServerName() rather than getLocalName(), this seems like a bug to me. Admittedly one that only causes problems in clustered & load balanced environments, but it's just affected our site so it would be great if it could be fixed for 5.2 final release. A final release of Tapestry should not have a bug like this in it!
Unless anyone has a convincing argument why it should be this way, of course...