Is this a private or a public feature? Can I use it in my custom widgets?

The panel Header is the only place where xhooks is used in Ext.

Code:
Ext.define('Ext.panel.Header', {
    ....
    initComponent: function() {
        ...
            me.titleCmp = new Ext.draw.Component({
                width     : 15,
                ariaRole  : 'heading',
                focusable : false,
                viewBox   : false,
                flex      : 1,
                id        : me.id + '_hd',
                autoSize  : true,
                margins   : '5 0 0 0',
                items     : [ me.textConfig ],
                xhooks: {
                    setSize: function (width) {
                        // don't pass 2nd arg (height) on to setSize or we break 'flex:1'
                        this.callParent([width]);
                    }
                },
                // this is a bit of a cheat: we are not selecting an element of titleCmp
                // but rather of titleCmp.items[0]
                childEls  : [
                    { name: 'textEl', select: '.' + me.baseCls + '-text' }
                ]
            });
        ....            
    },
    ...