Re: [manifest] Define identity of a web app. (#272)

> I don't see what that would achieve except create incompatibility between browsers, with some apps only working in some browsers.

Won't this happen already? Google will have different installability signals to us (and already do!) - the same will be true with any other vendor that implements this. The signals are not standard and could change from one release to another for whatever arbitrary reason (e.g., Moz might say we only require icons to be at least 90x90px, while Google might demand at least 144x144px... and a million other little variations from there... like must be PNG etc. while some other vendor might demand .jpeg and a splash screen).

> Saying that a manifest URL should be unique to each web app does not prevent OSs using their own identifiers, it just makes it possible to use manifest URL as an identifier and prevents duplicate conflicting apps trying to use the same manifest URL.

Sure, but there are still ways of working around this even with the proposed same origin restriction (`foo.com/a` and `foo.com/b` both use `foo.com/manifest`): so uniqueness is just a UA enforced thing irrespective of same origin... whichever app gets installed first is the one that wins. 

There is no real way to solve this problem, AFAICT, because even if we force the manifest to live in the scope of `foo.com/a`, the manifest may say `{"scope": "/"}`. This again just creates a race condition about which application first squats a URI space (in either manifest or in scope). 

> I have a question about that actually. If a manifest URL is used to update apps and two apps share the same manifest URL, what happens if the metadata of those two apps diverges?

They just diverge.

> How would a user agent update the manifest URL of an app to a different one and know that it's still the same app?

I'm not sure how you would update the manifest's URL (that doesn't change after install) - but you could always use the same manifest URL but just redirect it to the new URL, no? Then two apps could be redirected to different URLs. 

But let's pretend you could change the manifest URL for whatever reason, it wouldn't really matter AFAICT: it would just be a new source of information for arbitrary metadata. It would be like a site changing from one JS library to another JS library - so long as the APIs are the same, the app would continue to work (same in this theoretical case). This is because the OS controls what the real identifier for the application is - and the manifest just serves as data source for metadata, no? 

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