[Web Install] Make navigation.install do the right thing for same-origin installation

[dfabulich]

onbeforeinstallprompt has always had a weirdness in its API: you have to store a reference to the onbeforeinstallprompt event and use that event object to initiate the prompt.

Code sample from MDN:

let installPrompt = null;
const installButton = document.querySelector("#install");

window.addEventListener("beforeinstallprompt", (event) => {
  event.preventDefault();
  installPrompt = event;
  installButton.removeAttribute("hidden");
});
installButton.addEventListener("click", async () => {
  if (!installPrompt) {
    return;
  }
  const result = await installPrompt.prompt();
  console.log(`Install prompt was: ${result.outcome}`);
  installPrompt = null;
  installButton.setAttribute("hidden", "");
});

It would be nice if navigator.install() would do the right thing for same-origin installation as well, perhaps with no URL parameter in that case. That could change the sample to:

const installButton = document.querySelector("#install");

window.addEventListener("beforeinstallprompt", (event) => {
  event.preventDefault();
  installButton.removeAttribute("hidden");
});
installButton.addEventListener("click", () => {
  navigator.install().then( origin => console.log( `Installation started from ${origin}` ));
});

[diekus]

+1. We're looking into the implications of this. It'd be nice to have one consistent ergonomic API that would cover both installation types (same and cross domain).

[diekus]

We'd be keeping the same threshold of onbeforeinstallprompt for navigator.install() same domain installs, but plan at phasing out the former for the latter. Do let me know your thoughts, and if everything seems alright I'll proceed to close this issue.