extjs combobox下拉框显示位置问题

{
//baseCls:"x-plain",//强制与颜色 匹配
layout:"form",
labelWidth:55,
columnWidth:.14,
items:[{
xtype:"combo",
fieldLabel:'药品类别',
mode:'local',
store:new Ext.data.SimpleStore({
fields:['value','text'],
data:[['0','全部'],['1','西药'],['2','成药'],['3','草药'],['4','检查治疗项目'],['9','其他']]
}),
value:"全部",
triggerAction:"all",
valueField:'value',
displayField:'text',
width:100,
valueField:"value",
editable:false,
name:"midKind",
id:"midKind",
hiddenName:'midKind'
}]
}