Re: [w3c/manifest] Add a unique identifier for a PWA (#586)

> Feels to me like if we were building this from scratch, we would likely go with scope? It's the same as service workers, and less likely to change. Sounds like the main reason start_url is preferred is because we would only be changing expectations for one set of devs (Android), but I wonder if long term scope will make more sense for future developers. Is start_url even that intuitive/expected by Desktop developers today?

 I agree. The scope is a better choice if not considering change of expecations from one set of devs. A default value is supported mostly to allow smooth transition from existing web apps that don't have IDs. That's why we are favoring start_url as that's expected by existing desktop users.

 > I assume here that regardless of what we choose, already installed apps will be migrated to the new ID scheme? (i.e. on the first chrome version that has the new code, the ID'ing will be migrated to the new scheme, and updates to the app will continue working by matching using the new ID scheme). 


They will be migrated to the new ID scheme and if we are choosing the ID scheme that has a new id field, the default value will be used if the apps don't have it specified.


> As I understand it Firefox desktop does not support installing PWAs, so this analysis actually only covers Chrome desktop and Chrome for Android.

Oh thanks for pointing out, I just found out the announcement of them dropping the support recently. I will remove firefox desktop from the doc.

> My reading of this is "If we'd have thought about this earlier we may have concluded that manifest URL was the best identifier for a web app, but since Chrome used start URL as the identifier and we never told developers that manifest URLs shouldn't change, we're now going to have to specify what Chrome desktop already does, but with a hack to work around the fact that start URL was always a bad choice as it's the URL which is most likely to change."

I think this is misrepresenting what I was saying in the doc. The doc was comparing manifest_url vs a `id` field. With `id` not specified, `start_url` was preferred as the default value. But that's totally different from using start_url **as the id**. And apparently `id` is the most stable option. I also tried to [explain in detail ](https://github.com/philloooo/pwa-unique-id/blob/main/explainer.md#3-global_id--processed-manifest_url) that doing 301 redirect for use case of changing manifest_url either doesn't really support the use case, or ends up with unreliable behaviors for user agents. 

Thanks for listing out the current user agents and PWA stores. Most of the user agents you are listed are chromium based. 
As I stated in the doc, safari doesn't support updating/looking up installed PWAs, so it doesn't add any use cases relevant to the unique ID problem.
I'd be interested to know any use cases that I am missing in this doc.

PWA stores are being considered when comparing the  pros and cons of the options. I can elaborate more on specific examples of PWA stores in the beginning of the doc if that's helpful. 

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