The source code

    /**
     * @class Ext.Viewport
     * @extends Ext.Container
     * <p>A specialized container representing the viewable application area (the browser viewport).</p>
     * <p>The Viewport renders itself to the document body, and automatically sizes itself to the size of
     * the browser viewport and manages window resizing. There may only be one Viewport created
     * in a page. Inner layouts are available by virtue of the fact that all {@link Ext.Panel Panel}s
     * added to the Viewport, either through its {@link #items}, or through the items, or the {@link #add}
     * method of any of its child Panels may themselves have a layout.</p>
     * <p>The Viewport does not provide scrolling, so child Panels within the Viewport should provide
     * for scrolling if needed using the {@link #autoScroll} config.</p>
     * <p>An example showing a classic application border layout:</p><pre><code>
    new Ext.Viewport({
        layout: 'border',
        items: [{
            region: 'north',
            html: '&lt;h1 class="x-panel-header">Page Title&lt;/h1>',
            autoHeight: true,
            border: false,
            margins: '0 0 5 0'
        }, {
            region: 'west',
            collapsible: true,
            title: 'Navigation',
            width: 200
            // the west region might typically utilize a {@link Ext.tree.TreePanel TreePanel} or a Panel with {@link Ext.layout.AccordionLayout Accordion layout}
        }, {
            region: 'south',
            title: 'Title for Panel',
            collapsible: true,
            html: 'Information goes here',
            split: true,
            height: 100,
            minHeight: 100
        }, {
            region: 'east',
            title: 'Title for the Grid Panel',
            collapsible: true,
            split: true,
            width: 200,
            xtype: 'grid',
            // remaining grid configuration not shown ...
            // notice that the GridPanel is added directly as the region
            // it is not "overnested" inside another Panel
        }, {
            region: 'center',
            xtype: 'tabpanel', // TabPanel itself has no title
            items: {
                title: 'Default Tab',
                html: 'The first tab\'s content. Others may be added dynamically'
            }
        }]
    });
    </code></pre>
     * @constructor
     * Create a new Viewport
     * @param {Object} config The config object
     * @xtype viewport
     */
    Ext.Viewport = Ext.extend(Ext.Container, {
        /*
         * Privatize config options which, if used, would interfere with the
         * correct operation of the Viewport as the sole manager of the
         * layout of the document body.
         */
        /**
         * @cfg {Mixed} applyTo @hide
         */
        /**
         * @cfg {Boolean} allowDomMove @hide
         */
        /**
         * @cfg {Boolean} hideParent @hide
         */
        /**
         * @cfg {Mixed} renderTo @hide
         */
        /**
         * @cfg {Number} height @hide
         */
        /**
         * @cfg {Number} width @hide
         */
        /**
         * @cfg {Boolean} autoHeight @hide
         */
        /**
         * @cfg {Boolean} autoWidth @hide
         */
        /**
         * @cfg {Boolean} deferHeight @hide
         */
        /**
         * @cfg {Boolean} monitorResize @hide
         */

        initComponent : function() {
            Ext.Viewport.superclass.initComponent.call(this);
            document.getElementsByTagName('html')[0].className += ' x-viewport';
            this.el = Ext.getBody();
            this.el.setHeight = Ext.emptyFn;
            this.el.setWidth = Ext.emptyFn;
            this.el.setSize = Ext.emptyFn;
            this.el.dom.scroll = 'no';
            this.allowDomMove = false;
            this.autoWidth = true;
            this.autoHeight = true;
            Ext.EventManager.onWindowResize(this.fireResize, this);
            this.renderTo = this.el;
        },

        fireResize : function(w, h){
            this.fireEvent('resize', this, w, h, w, h);
        }
    });
    Ext.reg('viewport', Ext.Viewport);