Cytoscape Graph is not showing

Question

I am trying to show a cytoscape graph in an ext window. Everything is working well, until i add the ext-all.css, the graph is no longer displayed.

This is the panel were the graph is supposed to be shown ( the #cy div)

 var formPanel2 = new Ext.form.FormPanel({
            id : 'graph_model',
            bodyStyle : 'padding:10px',
            width :400,
            height : 400,
            overflow:'auto',
            items : [ {
                    html: "<div id='cy' style=' height: 100%; width: 100%;  position: absolute; left: 0; top: 0 '></div>",
                    xtype: "panel"} ]
  });

where is the problem ?