表格的列增加locked属性后不再响应itemdblclick

如题,
同样的代码不加locked属性是响应的;var data =[[1,"000811121267","Jacky",185.37,207.79],
           [1,"000811121268","Tommy",185.37,207.79],
           [1,"000811121269","Lucy",185.37,207.79]];
var store = new Ext.data.SimpleStore({data:data, fields:["id","no","name", "day01", "day02"]});
var myGrid = Ext.create('Ext.grid.Panel',{
           renderTo:"hello",
           title:"click with locked",
           width: 300,
           height: 150,
           columns:[
                    {header:"number", dataIndex:"no"},
                    {header:"name", dataIndex:"name"},
                    {header:"day1", dataIndex:"day01", width:70},
                    {header:"day2", dataIndex:"day02", width:70}],
           store:store,
           listeners:{
               'itemdblclick' : function(){alert(7);}    //works!
           }
           });加了locked, 比如{header:"number", dataIndex:"no", locked:true}就不再响应了,
这是为神马呢?