it seems that its because of warp:true
trying this code can solve the problem.
<div id="topic-grid" style="border:1px solid #99bbe8;overflow: hidden; width: 665px; height: 300px;"><div id="new-topic-grid" style="width: 100%; height: 100%"></div></div>
// create the editor grid
var grid = new Ext.grid.Grid('new-topic-grid', {
ds: ds,
cm: cm,
selModel: new Ext.grid.RowSelectionModel({singleSelect:true}),
enableColLock:false,
loadMask: true
});
// make the grid resizable, do before render for better performance
var rz = new Ext.Resizable('topic-grid', {
//wrap:true,
minHeight:100,
pinned:true,
handles: 's'
});