https://w3c.github.io/ServiceWorker/#client-navigate is synchronously accessing to a client active service worker

Step 4 of the navigate algorithm:
If this's associated service worker client's active service worker is not this's relevant global object’s service worker, return a promise rejected with a TypeError.

This step is done synchronously in the service worker's thread.
I would expect this check to be done within a 'run in parallel' block since it is difficult to know from a service worker whether a client is controlled or not.

Start with the navigate algorithm at the linked Service Worker specification and read Step 4 in its surrounding context. Investigate how the active service worker check is performed from a service worker thread; done means the specification resolves the issue's concern about synchronous access to the client state.