[manifest] Allow developers to detect when the user has added the site to their home screen (or equivalent) (#417)

I've heard some feedback from developers that they are very excited by encouraging users to press the 'Add to Home Screen' button in Chrome, and the equivalents in Opera etc, but that it is a problem that they can't track that event for analytics purposes

It would be great if we could somehow expose to developers that the user has installed their web app via some UA-provided button

We could create a new event for this, or one alternate idea would be to fire a non-cancellable [BeforeInstallPrompt](https://developermozillaorg/en-US/docs/Web/API/BeforeInstallPromptEvent) event which immediately resolves the `userChoice ` promise whenever the user presses a UA-provided button to install the web app

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/417