It listens to the online and offline events (using https://github.com/chrisbolin/react-detect-offline which is really cool). You really have to turn off your network connection.

Or fire the event yourself using window.dispatchEvent(new Event('offline'))

Worked for me in Chrome 60.0.3112.101

I am on Chrome 60.0.3112.101 too and it works.

Firefox 54 on linux, didn't work.