Fixed bug in throttling code.

IPython/html/static/notebook/js/widgetmanager.js

@@ -160,9 +160,13 b''
         WidgetManager.prototype.callbacks = function (view) {
             // callback handlers specific a view
             var callbacks = {};
-            var cell = view.options.cell;
-            if (cell !== null) {
+            if (view !== undefined && 
+                view !== null && 
+                view.options.cell !== undefined && 
+                view.options.cell !== null) {
+
                 // Try to get output handlers
+                var cell = view.options.cell;
                 var handle_output = null;
                 var handle_clear_output = null;
                 if (cell.output_area !== undefined && cell.output_area !== null) {

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

@@ -56,6 +56,7 b' function(WidgetManager, Underscore, Backbone){'
             if (this.comm !== undefined) {
                 var data = {method: 'custom', content: content};
                 this.comm.send(data, callbacks);
+                this.pending_msgs++;
             }
         },
 
@@ -158,7 +159,13 b' function(WidgetManager, Underscore, Backbone){'
 
             // Only sync if there are attributes to send to the back-end.
             if (_.size(attrs) > 0) {
-                var callbacks = options.callbacks || {};
+
+                // If this message was sent via backbone itself, it will not
+                // have any callbacks.  It's important that we create callbacks
+                // so we can listen for status messages, etc...
+                var callbacks = options.callbacks || this.callbacks();
+
+                // Check throttle.
                 if (this.pending_msgs >= this.msg_throttle) {
                     // The throttle has been exceeded, buffer the current msg so
                     // it can be sent once the kernel has finished processing 
@@ -181,8 +188,7 b' function(WidgetManager, Underscore, Backbone){'
 
                 } else {
                     // We haven't exceeded the throttle, send the message like 
-                    // normal.  If this is a patch operation, just send the 
-                    // changes.
+                    // normal.
                     var data = {method: 'backbone', sync_data: attrs};
                     this.comm.send(data, callbacks);
                     this.pending_msgs++;

IPython/html/static/notebook/js/widgets/widget_string.js

@@ -176,7 +176,7 b' define(["notebook/js/widgets/widget"], function(WidgetManager){'
         },
         
         handleChanging: function(e) { 
-            // Handles and validates user input.
+            // Handles user input.
             
             // Calling model.set will trigger all of the other views of the 
             // model to update.