Multiple $watch can be a performance drag

[wjaspers]

On the off-chance someone is using this more than once in the page, this adds more $watch functions than it needs. Only one is necessary, and only a single change detection can trigger the diff. Near simultaneous changes to left and right would otherwise create separate digest cycles.

Newer angular standards/preferences would suggest binding left and right to the controller, but for the sake of the current code, here's all that's needed.

var _this = this;
var unbindWatch = $scope.$watch(function () {
   return { left: $scope.left, right: $scope.right };
}, function (newValue, oldValue, localScope) {
  _this.doDiff();
}, true); 
$scope.$on("$destroy", unbindWatch);

[bill-long]

I'm not really maintaining this anymore. If you want to submit a PR, I'll merge it in, though it looks like the change above is only to the .js file? A change to the .ts would be preferred, and then the .js can just be compiled from it.

But I'd be happy for anyone actively using this to take over.