I've done this before, its a lot of fun, you learn a lot about the browser. Especially caching. If you ever suspect caching issues for a bug / issue, open up Chrome Dev Tools, go to the "Application" tab, then "Storage" if you click the Clear site data button, it will wipe all cache, then right click on the refresh button, and "Empty cache and hard reset" if you do both of these (though one's probably enough) you will start over with nothing cached. Cache becomes very tricky when building a PWA.

I don't know if its been fixed since, but I really needed to tell if I was offline, only way was to make requests, if they failed assume offline, keep retrying till online to sync data to backend.