Add order respecting method

IPython/html/static/widgets/js/widget.js

@@ -337,7 +337,7 function(WidgetManager, _, Backbone){
             return null;
         },
 
-        do_diff: function(old_list, new_list, removed_callback, added_callback) {
+        do_diff: function(old_list, new_list, removed_callback, added_callback, respect_order) {
             // Difference a changed list and call remove and add callbacks for 
             // each removed and added item in the new list.
             //
@@ -349,8 +349,29 function(WidgetManager, _, Backbone){
             //      Callback that is called for each item removed.
             // added_callback : Callback(item)
             //      Callback that is called for each item added.
+            // [respect_order] : bool [True]
+            //      Whether or not the order of the list matters.
+
+            if (respect_order || respect_order===undefined) {
+                // Walk the lists until an unequal entry is found.
+                var i;
+                for (i = 0; i < new_list.length; i++) {
+                    if (i < old_list.length || new_list[i] !== old_list[i]) {
+                        break;
+                    }
+                }
 
+                // Remove the non-matching items from the old list.
+                for (var j = i; j < old_list.length; j++) {
+                    console.log(j, old_list.length, old_list[j]);
+                    removed_callback(old_list[j]);
+                }
 
+                // Add the rest of the new list items.
+                for (i; i < new_list.length; i++) {
+                    added_callback(new_list[i]);
+                }
+            } else {
             // removed items
             _.each(this.difference(old_list, new_list), function(item, index, list) {
                 removed_callback(item);
@@ -360,6 +381,7 function(WidgetManager, _, Backbone){
             _.each(this.difference(new_list, old_list), function(item, index, list) {
                 added_callback(item);
             }, this);
+            }
         },
 
         difference: function(a, b) {