it would have been possible without locks if the client would have sent IPC messages to the other threads event queue and then wait until the result comes back.

it would have been possible without locks if the client would have sent IPC messages to the other threads event queue and then wait until the result comes back. This way all access to the GUI widgets is serialized via the event loop. In windows they had apartment model COM objects that used to work by this principle (ouch, i feel so old now ...)