It's not clear to me if you can really just solve this like you can with the terminal case. The thing with a terminal is that it's insanely easy to fork streams

It's not clear to me if you can really just solve this like you can with the terminal case. The thing with a terminal is that it's insanely easy to fork streams -- you can just interleave the streams, and even without active cooperation between the processes you still get a behavior that has some semblance of semantic sense. But with a GUI system, you're not just dealing with a pipeline; you're dealing with a state machine. Do you really take a similar approach? When a window receives a message/event, which process responds? If multiple processes respond, which message/event gets sent and which gets discarded? If only one responds, does the same process also get the reply to the reply, or might the fork get the reply instead? You could interleave events randomly here too, but it seems like far less sensible of a solution than with the terminal case.