session-desktop/js/focus_listener.js

(function () {
  'use strict';

  var windowFocused;
  window.addEventListener('blur', function() {
    windowFocused = false;
  });
  window.addEventListener('focus', function() {
    windowFocused = true;
  });

  window.isFocused = function() {
    return windowFocused;
  };

})();