"You probably have multiple tabs open" ☂ bug

[patrickhulce]

Within Lighthouse, this error comes from driver.assertNoSameOriginServiceWorkerClients. This is called as the first step in setting up the driver to start loading your page and fetches the list of service workers currently controlling the page to check if it is safe to unregister.

If there are pages other than the current one that are being controlled by the same service worker, we won't be able to unregister the worker to obtain accurate results, and so we prevent the run.

There are a few cases in which this error shows up.

1. You have more than one tab open to the same origin.
   Workaround: close the other tabs to the origin you have open and re-run the audit.
2. You have another tab open that has an iframe to the same origin.
   Workaround: close the other tab and re-run the audit. This can be more difficult to detect and is easier to run Lighthouse in a clean profile that has no other tabs open.
   Notes: most commonly a YouTube page since sites frequently embed the YouTube player
3. The target page has an iframe that is also controlled by the service worker.
   Workaround: use the CLI or Chrome Canary to audit the page.
   Notes: tracked by https://bugs.chromium.org/p/chromium/issues/detail?id=750767, fixed in Chromium 61.0.3163.39
4. The target page has a link[rel=prerender] element to a page controlled by the same service worker.
   Workaround: use the CLI or wait for the prerender page to die (you can see the currently prerendered page at chrome://net-internals/#prerender)
   Example issue: Fails on page with prerender resource hint. #1335

[paulirish]

Let's add to help/faq docs, same as #2784 (comment)

[connorjclark]

We should see if we can remove this check/clearing the service worker data, and instead use signals from installability errors on protocol to check if install will work.