Preserve sec-fetch-dest for passed navigate requests

[ErikWitt]

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.

[asutherland]

I understand the problem here to be that the destination is not propagated by the Request constructor as invoked by fetch(). This probably should be discussed in the fetch spec, although any changes might eventually involve the ServiceWorkers spec.

whatwg/fetch#1323 seems like it may be the most directly related open issue, with whatwg/fetch#1762 touching on it tangentially. @annevk probably has more useful thoughts.

[annevk]

It's hard as the service worker can replay these responses anywhere. There's no guarantee that a response to a navigation won't be delivered to a <script>. #1395 has some musings, but I think there are even earlier musings on this topic that go a bit more into the challenges.

[wanderview]

Random brainstorming: The request could track the original navigation internally. If a fetch is made with the request while the original navigation is outstanding, then sec-fetch-dest is passed through. Otherwise is reverts to empty string.

This would be hard to understand and the request would no longer really be immutable, though.