The source code

    /**
     * @class Ext.layout.MenuLayout
     * @extends Ext.layout.ContainerLayout
     * <p>Layout manager used by {@link Ext.menu.Menu}. Generally this class should not need to be used directly.</p>
     */
     Ext.layout.MenuLayout = Ext.extend(Ext.layout.ContainerLayout, {
        monitorResize : true,

        type: 'menu',

        setContainer : function(ct){
            this.monitorResize = !ct.floating;
            // This event is only fired by the menu in IE, used so we don't couple
            // the menu with the layout.
            ct.on('autosize', this.doAutoSize, this);
            Ext.layout.MenuLayout.superclass.setContainer.call(this, ct);
        },

        renderItem : function(c, position, target){
            if (!this.itemTpl) {
                this.itemTpl = Ext.layout.MenuLayout.prototype.itemTpl = new Ext.XTemplate(
                    '<li id="{itemId}" class="{itemCls}">',
                        '<tpl if="needsIcon">',
                            '<img alt="{altText}" src="{icon}" class="{iconCls}"/>',
                        '</tpl>',
                    '</li>'
                );
            }

            if(c && !c.rendered){
                if(Ext.isNumber(position)){
                    position = target.dom.childNodes[position];
                }
                var a = this.getItemArgs(c);

    //          The Component's positionEl is the <li> it is rendered into
                c.render(c.positionEl = position ?
                    this.itemTpl.insertBefore(position, a, true) :
                    this.itemTpl.append(target, a, true));

    //          Link the containing <li> to the item.
                c.positionEl.menuItemId = c.getItemId();

    //          If rendering a regular Component, and it needs an icon,
    //          move the Component rightwards.
                if (!a.isMenuItem && a.needsIcon) {
                    c.positionEl.addClass('x-menu-list-item-indent');
                }
                this.configureItem(c);
            }else if(c && !this.isValidParent(c, target)){
                if(Ext.isNumber(position)){
                    position = target.dom.childNodes[position];
                }
                target.dom.insertBefore(c.getActionEl().dom, position || null);
            }
        },

        getItemArgs : function(c) {
            var isMenuItem = c instanceof Ext.menu.Item,
                canHaveIcon = !(isMenuItem || c instanceof Ext.menu.Separator);

            return {
                isMenuItem: isMenuItem,
                needsIcon: canHaveIcon && (c.icon || c.iconCls),
                icon: c.icon || Ext.BLANK_IMAGE_URL,
                iconCls: 'x-menu-item-icon ' + (c.iconCls || ''),
                itemId: 'x-menu-el-' + c.id,
                itemCls: 'x-menu-list-item ',
                altText: c.altText || ''
            };
        },

        //  Valid if the Component is in a <li> which is part of our target <ul>
        isValidParent : function(c, target) {
            return c.el.up('li.x-menu-list-item', 5).dom.parentNode === (target.dom || target);
        },

        onLayout : function(ct, target){
            Ext.layout.MenuLayout.superclass.onLayout.call(this, ct, target);
            this.doAutoSize();
        },

        doAutoSize : function(){
            var ct = this.container, w = ct.width;
            if(ct.floating){
                if(w){
                    ct.setWidth(w);
                }else if(Ext.isIE9m){
                    ct.setWidth(Ext.isStrict && (Ext.isIE7 || Ext.isIE8 || Ext.isIE9) ? 'auto' : ct.minWidth);
                    var el = ct.getEl(), t = el.dom.offsetWidth; // force recalc
                    ct.setWidth(ct.getLayoutTarget().getWidth() + el.getFrameWidth('lr'));
                }
            }
        }
    });
    Ext.Container.LAYOUTS['menu'] = Ext.layout.MenuLayout;