set screen.index.

lib/widgets/node.js

@@ -33,7 +33,7 @@ function Node(options) {
   this.children = [];
   this.$ = this._ = this.data = {};
   this.uid = Node.uid++;
-  this.index = -1;
+  this.index = this.index != null ? this.index : -1;
 
   if (this.type !== 'screen') {
     this.detached = true;

lib/widgets/screen.js

@@ -200,6 +200,7 @@ Screen.bind = function(screen) {
 
   if (!~Screen.instances.indexOf(screen)) {
     Screen.instances.push(screen);
+    screen.index = Screen.total;
     Screen.total++;
   }