[editor] no undo applicable on initial content, fix #2287

vispa/extensions/codeeditor/static/js/editor.js

@@ -88,6 +88,8 @@ define(["jquery", "emitter", "./action", "require", "ace/ace", "ace/ext-language
 
     setContent: function(content) {
       this.ace.setValue(content);
+      var UndoManager = require("ace/undomanager").UndoManager;
+      this.ace.getSession().setUndoManager(new UndoManager());
       return this;
     },