Scroll and Expand are not working properly

We have one grid panel with in ext.window... the scroll bar of gridpanel is automatically moving up while scrolling it is't working properly and when ext.window is expanded the grid panel is not expanding..i guess some properties has to be set??..will using autoExpandColumn in gridpanel will solve the problem??



extWin=new Ext.Window({

title:"Locate Managed Object",items:[new Ext.grid.GridPanel({

title: "Managed Elements",

region: "center",

height:250,

width:500, renderTo:"tree-id",

viewConfig: {forceFit: true},

store: store,

sm: new GeoExt.grid.FeatureSelectionModel({selectControl:select}),

cm: new Ext.grid.ColumnModel({

defaults: {

sortable: true

},

columns: [

{header:"Managed Elements",dataIndex:"fid"}

]

})

})]

});

extWin.show();