Preserve sec-fetch-dest for passed navigate requests

For navigate requests the sec-fetch-dest header is set to document. Once a service worker is introduced that simply forwards the request to the network, the sec-fetch-dest header changes to empty.

There are systems that rely on that header on server side to detect navigate requests and act accordingly. Shopify for example has some language/currency persistence logic that uses it.

Introducing a service worker that forwards requests (e.g. to cache responses for offline handling), breaks functionality.

I would argue that a navigate request from the browser should behave the same when proxied through a service worker. If this cannot be assumed by the developer, it gets very hard to implement service worker without potentially breaking functionality.

To have a reproducable example, here is a test website: https://dm-clone-optimized.app.baqend.com/fetch-dest/

It installs a service worker and when reloading you can see the sec-fetch-dest header being different depending on whether you bypass the service worker or not.

Start with the reproducible example at https://dm-clone-optimized.app.baqend.com/fetch-dest/ and compare the sec-fetch-dest header with and without the service worker. Read the Service Worker specification sections covering navigate requests and forwarding to the network; done means reaching a clarified or specified behavior that preserves the expected navigation semantics.