Currently the ServiceWorker specification does not specify how it interacts with non-"fully active" (bfcached) documents/window clients.

• Iterating over all clients does not filter out window clients with non-fully active documents. Example: Clients.get(), Clients.matchAll(), Clients.claim()
• Actions on window clients whose document is no longer fully active can still happen. Example: Client.postMessage()

We recommend following the WIP guide we've made on handling non-fully active documents, by adding "fully active" checks so that we won't include clients with non-fully active documents, and if necessary trigger discarding of the document (e.g. for Clients.claim()). This is already how it behaves in Chrome's implementation of BFCache, and @hiroshige-g will also publish some WPTs soon to show how it behaves in various browsers.

cc @domenic @fergal @altimin