diff --git a/core/mutator.js b/core/mutator.js
index 1bd6d0988..8379a79f4 100644
--- a/core/mutator.js
+++ b/core/mutator.js
@@ -352,6 +352,8 @@ Blockly.Mutator.prototype.setVisible = function(visible) {
     this.resizeBubble_();
     // When the mutator's workspace changes, update the source block.
     this.workspace_.addChangeListener(this.workspaceChanged_.bind(this));
+    // Update the source block immediately after the bubble becomes visible.
+    this.updateWorkspace_();
     this.applyColour();
   } else {
     // Dispose of the bubble.
@@ -371,18 +373,24 @@ Blockly.Mutator.prototype.setVisible = function(visible) {
 };
 
 /**
- * Update the source block when the mutator's blocks are changed.
- * Bump down any block that's too high.
  * Fired whenever a change is made to the mutator's workspace.
  * @param {!Blockly.Events.Abstract} e Custom data for event.
  * @private
  */
 Blockly.Mutator.prototype.workspaceChanged_ = function(e) {
-  if (e.isUiEvent ||
-      (e.type == Blockly.Events.CHANGE && e.element == 'disabled')) {
-    return;
+  if (!(e.isUiEvent ||
+      (e.type == Blockly.Events.CHANGE && e.element == 'disabled') ||
+      e.type == Blockly.Events.CREATE)) {
+    this.updateWorkspace_();
   }
+};
 
+/**
+ * Updates the source block when the mutator's blocks are changed.
+ * Bump down any block that's too high.
+ * @private
+ */
+Blockly.Mutator.prototype.updateWorkspace_ = function() {
   if (!this.workspace_.isDragging()) {
     var blocks = this.workspace_.getTopBlocks(false);
     var MARGIN = 20;