created a property that keeps track of whether the browser tab is visible, integrated it into audio view for John Travoltage, see https://github.com/phetsims/joist/issues/421
created a property that keeps track of whether the browser tab is visible, integrated it into audio view for John Travoltage, see https://github.com/phetsims/joist/issues/421

phetsims/joist,phetsims/joist,phetsims/joist