Call the code cell’s clear_output so a cell event is triggered.

Thanks to @gnestor for this, from PR #2394.

notebook/static/notebook/js/codecell.js

@@ -313,7 +313,7 @@ define([
             stop_on_error = true;
         }
 
-        this.output_area.clear_output(false, true);
+        this.clear_output(false, true);
         var old_msg_id = this.last_msg_id;
         if (old_msg_id) {
             this.kernel.clear_callbacks_for_msg(old_msg_id);
@@ -509,10 +509,10 @@ define([
     };
 
 
-    CodeCell.prototype.clear_output = function (wait) {
-        this.output_area.clear_output(wait);
-        this.set_input_prompt();
+    CodeCell.prototype.clear_output = function (wait, ignore_queue) {
         this.events.trigger('clear_output.CodeCell', {cell: this});
+        this.output_area.clear_output(wait, ignore_queue);
+        this.set_input_prompt();
     };
 
 

notebook/static/notebook/js/outputarea.js

@@ -942,7 +942,7 @@ define([
     };
 
 
-    OutputArea.prototype.clear_output = function(wait, ignore_que) {
+    OutputArea.prototype.clear_output = function(wait, ignore_clear_queue) {
         if (wait) {
 
             // If a clear is queued, clear before adding another to the queue.
@@ -955,7 +955,7 @@ define([
 
             // Fix the output div's height if the clear_output is waiting for
             // new output (it is being used in an animation).
-            if (!ignore_que && this.clear_queued) {
+            if (!ignore_clear_queue && this.clear_queued) {
                 var height = this.element.height();
                 this.element.height(height);
                 this.clear_queued = false;