Game stops when switching tabs

I've made myself an idle game to be played on a browser (HTML5) but the game stops when I switch tabs and no progress is made (it's an idle game).

I've played several games like that in the past and as a Player I would just leave the game open in it's own window without any other tab. But now as a developer, I've been requested several times to fix it and I've been incapable of doing so.

Does anybody happen to know how to keep the game running even with switching tabs?

Thanks in advance!