$timeout not firing when window isn't in focus?

Question

As part of learning AngularJS, I decided to rewrite Cookie Clicker using it. I have a controller set up and it has (among other things) $scope.Cookies, $scope.CPS (to represent the cookies per second), and an Update function like so:

$scope.Update = function() {
    $scope.Cookies = $scope.Cookies + ($scope.CPS / 100);

    $timeout($scope.Update, 10);
}

While I have the tab up front and center, the Cookie count increases correctly. I've found that if I'm looking at a different tab then this isn't updating.When switch back to my tab, it picks up where it left off. I've tested this by putting the Cookie count in the title bar and confirming that it stops. EDIT: Instead of running 100 times a second, it appears to be running 1 time a minute.

What can I do to ensure the timeout continues to fire at the expected interval?