A request's "window" is never a Window

[jyasskin]

Should request be allowed to use feature? has

1. Set window to request’s window.
2. If window is not a Window, return false.

However, request's window is '("no-window", "client", or an environment settings object whose global object is a Window object)'. That is, it can be an environment settings object, but it can't be a Window. This is confusing, but makes the algorithm here wrong.

[annevk]

Yeah, I'd recommend using

If window is "no-window", then return false.

if you really mean that. But note that window can be set for requests that pass through a service worker. If you really care about the environment that's currently making the request, you want client.