Disable break/return blocks in addition to warning.

blocks/procedures.js

@@ -843,6 +843,9 @@ Blockly.Blocks['procedures_ifreturn'] = {
    * @this Blockly.Block
    */
   onchange: function(e) {
+    if (this.workspace.isDragging()) {
+      return;  // Don't change state at the start of a drag.
+    }
     var legal = false;
     // Is the block nested in a procedure?
     var block = this;
@@ -868,8 +871,14 @@ Blockly.Blocks['procedures_ifreturn'] = {
         this.hasReturnValue_ = true;
       }
       this.setWarningText(null);
+      if (!this.isInFlyout) {
+        this.setDisabled(false);
+      }
     } else {
       this.setWarningText(Blockly.Msg.PROCEDURES_IFRETURN_WARNING);
+      if (!this.isInFlyout && !this.getInheritedDisabled()) {
+        this.setDisabled(true);
+      }
     }
   },
   /**