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What I am trying to do now is implement HTTP/2/HTTP/3 on that connection that behaves the same as between a ServiceWorker and a WindowClient on Chromium, which as far I can see is the only shipped implementation of full-duplex streaming using fetch() in a browser; for example typing lowercase letters in this plnkr
Half duplex stream streams the response to the client without waiting for the WritableStream side of a TransformStream to be closed.