Remove typing cursor from combobox
// example combobox config
xtype: 'combo',
allowBlank: false,
forceSelection: true,
valueField:'id',
displayField:'name',
store: myStore,

// add this "onFocus" config
onFocus: function() {
    var me = this;

    if (!me.isExpanded) {
        me.expand()
    }
    me.getPicker().focus();
},