extjs树面板。如何带来前方

Question

如何在extjs中将树面板放在前面？ 当我单击并打开树面板时，他位于另一个组件下。我该如何解决这个问题？

...showMetricPicker : function(metricField) {
        if (this.metricPicker == null) {
            this.metricPicker = Ext.create('MetricPicker', {
                renderTo: this.getEl(),
                style: {
                    zIndex: 99999
                },
                width: 400,
                height: 300,
                border: true,
                selModel: {
                    mode: 'SINGLE',
                    listeners: {
                        beforeselect: function(tree, node) {
                            //allow only metric selection not group
                            return node.get('leaf');
                        }
                    }
                },
                listeners: {
                    scope: this,
                    itemdblclick : function(view, record, item, index, e, eOpts) {
                        this.metricDblClick(record);
                    }
                }
            });
            this.metricPicker.btnShowCollectingMetrics.setVisible(false);

            this.metricPicker.btnShowCommonMetrics.setVisible(false);
            this.stopEventOnMouseDown(this.metricPicker);
        } else {
            this.metricPicker.show();
        }
        this.metricPicker.metricField = metricField;
        this.metricPicker.getEl().alignTo(metricField.getEl(), 'tl-tl');.....