Re: [w3c/manifest] Allowing only site-triggered install prompt (#627)

Hi, sorry I've missed this discussion until today. This is something I think about all the time.

I do mostly agree with @photopea and I think the Google Chrome team has been gradually coming around to the position that developers should be in control of this, not the UA. We've gone from a relatively unpredictable engagement requirement (requiring heavy use of the site before the prompt can trigger) to an almost instantaneous one: we now deliver the BIP event after a few seconds of interactivity with a site. This means if the user immediately closes a site, they won't see it, but any normal usage will be able to show the prompt.

We should recognise two separate issues:

* the _static_ issue of which sites are eligible (manifest, correct icon size, fetch handler, etc, etc), and
* the _dynamic_ issue of under what circumstances do we allow the prompt (after the user has engaged with the site by a certain amount).

I am not concerned about the dynamic issue, because it doesn't present a compatibility risk. Different browsers can have different dynamic (engagement) requirements, and it doesn't "break" sites because the developer doesn't need to design differently. You just need to know that you may need to wait awhile before the prompt event comes up.

I *am* concerned about the static issue. I think it's a web compat problem if different user agents have different requirements for a site to be eligible to receive a prompt, because then, as you say, we're in the 90s world where developers need to test that they meet the criteria for each major browser. That harms us (the browser manufacturer) too, because then we can't change our policy in the future. (Just as in the 2000s, IE was ultimately harmed by being tied to its own de facto standards.)

> User agent should not be the "robot", who decides things for us.

In some sense. The UA will always play a role in deciding things on behalf of the user. After all, they are the user *agent* (authorized by the user to act on their behalf in making certain decisions). That's why I am OK with requiring a minimum engagement, and with having a browser-specific policy on when you can re-show a prompt if it has been rejected by the user. As well as, of course, UA-specific UI for manually installing the site without a site-triggered prompt.

What's weird about this is that we allow sites to spam notification permission requests immediately (which arguably is requesting a more invasive capability than installation), but we're more conservative with showing the install prompt. I think we should be more consistent about this.

> Web authors would have to test their product with many borwsers to make sure it works everywhere. We are getting the web back to 90s.

Yep, I definitely share this concern, as stated above.

Chrome has tried to make the "whether or not you are a PWA" check as transparent as possible, publishing the [checklist](https://developers.google.com/web/progressive-web-apps/checklist) and providing a [tool](https://chrome.google.com/webstore/detail/lighthouse/blipmdconlkpinefehnmjammfjpmpbjk) to verify whether your site is eligible. But up until now, this has been a proprietary Chrome feature.

I definitely think we'll want to lock this in at the specification level at some point, so the Lighthouse tool can tell you definitively whether your site meets the _specified_ requirement for installability in all browsers. However, I think at the moment we're still early on in the life cycle of this feature, and people are worried if we codify this, we won't be able to increase the requirements later (e.g., by enforcing that your fetch handler actually serves content).

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