The Game Hidden Event.
This event is dispatched by the Game Visibility Handler when the document in which the Game instance is embedded enters a hidden state. Only browsers that support the Visibility API will cause this event to be emitted.
In most modern browsers, when the document enters a hidden state, the Request Animation Frame and setTimeout, which control the main game loop, will automatically pause. There is no way to stop this from happening. It is something your game should account for in its own code, should the pause be an issue (i.e. for multiplayer games)