It appears that Quantum takes full control of Eclipse when making a connection. This is usually not noticeable, but when connecting to a server with a lot of network lag (or, some issue that only fails by virtue of a timeout), Eclipse cannot be used while the connection is opening.
Ideally, the connection would spawn a thread.
Testing this is difficult. One way would be to try connecting to a server that isn't there, or behind a firewall that just eats the packets.