The source code

    /**
     * @class Ext.layout.boxOverflow.Menu
     * @extends Ext.layout.boxOverflow.None
     * Description
     */
    Ext.layout.boxOverflow.Menu = Ext.extend(Ext.layout.boxOverflow.None, {
        /**
         * @cfg afterCls
         * @type String
         * CSS class added to the afterCt element. This is the element that holds any special items such as scrollers,
         * which must always be present at the rightmost edge of the Container
         */
        afterCls: 'x-strip-right',

        /**
         * @property noItemsMenuText
         * @type String
         * HTML fragment to render into the toolbar overflow menu if there are no items to display
         */
        noItemsMenuText : '<div class="x-toolbar-no-items">(None)</div>',

        constructor: function(layout) {
            Ext.layout.boxOverflow.Menu.superclass.constructor.apply(this, arguments);

            /**
             * @property menuItems
             * @type Array
             * Array of all items that are currently hidden and should go into the dropdown menu
             */
            this.menuItems = [];
        },

        /**
         * @private
         * Creates the beforeCt, innerCt and afterCt elements if they have not already been created
         * @param {Ext.Container} container The Container attached to this Layout instance
         * @param {Ext.Element} target The target Element
         */
        createInnerElements: function() {
            if (!this.afterCt) {
                this.afterCt  = this.layout.innerCt.insertSibling({cls: this.afterCls},  'before');
            }
        },

        /**
         * @private
         */
        clearOverflow: function(calculations, targetSize) {
            var newWidth = targetSize.width + (this.afterCt ? this.afterCt.getWidth() : 0),
                items    = this.menuItems;

            this.hideTrigger();

            for (var index = 0, length = items.length; index < length; index++) {
                items.pop().component.show();
            }

            return {
                targetSize: {
                    height: targetSize.height,
                    width : newWidth
                }
            };
        },

        /**
         * @private
         */
        showTrigger: function() {
            this.createMenu();
            this.menuTrigger.show();
        },

        /**
         * @private
         */
        hideTrigger: function() {
            if (this.menuTrigger != undefined) {
                this.menuTrigger.hide();
            }
        },

        /**
         * @private
         * Called before the overflow menu is shown. This constructs the menu's items, caching them for as long as it can.
         */
        beforeMenuShow: function(menu) {
            var items = this.menuItems,
                len   = items.length,
                item,
                prev;

            var needsSep = function(group, item){
                return group.isXType('buttongroup') && !(item instanceof Ext.Toolbar.Separator);
            };

            this.clearMenu();
            menu.removeAll();

            for (var i = 0; i < len; i++) {
                item = items[i].component;

                if (prev && (needsSep(item, prev) || needsSep(prev, item))) {
                    menu.add('-');
                }

                this.addComponentToMenu(menu, item);
                prev = item;
            }

            // put something so the menu isn't empty if no compatible items found
            if (menu.items.length < 1) {
                menu.add(this.noItemsMenuText);
            }
        },

        /**
         * @private
         * Returns a menu config for a given component. This config is used to create a menu item
         * to be added to the expander menu
         * @param {Ext.Component} component The component to create the config for
         * @param {Boolean} hideOnClick Passed through to the menu item
         */
        createMenuConfig : function(component, hideOnClick){
            var config = Ext.apply({}, component.initialConfig),
                group  = component.toggleGroup;

            Ext.copyTo(config, component, [
                'iconCls', 'icon', 'itemId', 'disabled', 'handler', 'scope', 'menu'
            ]);

            Ext.apply(config, {
                text       : component.overflowText || component.text,
                hideOnClick: hideOnClick
            });

            if (group || component.enableToggle) {
                Ext.apply(config, {
                    group  : group,
                    checked: component.pressed,
                    listeners: {
                        checkchange: function(item, checked){
                            component.toggle(checked);
                        }
                    }
                });
            }

            delete config.ownerCt;
            delete config.xtype;
            delete config.id;

            return config;
        },

        /**
         * @private
         * Adds the given Toolbar item to the given menu. Buttons inside a buttongroup are added individually.
         * @param {Ext.menu.Menu} menu The menu to add to
         * @param {Ext.Component} component The component to add
         */
        addComponentToMenu : function(menu, component) {
            if (component instanceof Ext.Toolbar.Separator) {
                menu.add('-');

            } else if (Ext.isFunction(component.isXType)) {
                if (component.isXType('splitbutton')) {
                    menu.add(this.createMenuConfig(component, true));

                } else if (component.isXType('button')) {
                    menu.add(this.createMenuConfig(component, !component.menu));

                } else if (component.isXType('buttongroup')) {
                    component.items.each(function(item){
                         this.addComponentToMenu(menu, item);
                    }, this);
                }
            }
        },

        /**
         * @private
         * Deletes the sub-menu of each item in the expander menu. Submenus are created for items such as
         * splitbuttons and buttongroups, where the Toolbar item cannot be represented by a single menu item
         */
        clearMenu : function(){
            var menu = this.moreMenu;
            if (menu && menu.items) {
                menu.items.each(function(item){
                    delete item.menu;
                });
            }
        },

        /**
         * @private
         * Creates the overflow trigger and menu used when enableOverflow is set to true and the items
         * in the layout are too wide to fit in the space available
         */
        createMenu: function() {
            if (!this.menuTrigger) {
                this.createInnerElements();

                /**
                 * @private
                 * @property menu
                 * @type Ext.menu.Menu
                 * The expand menu - holds items for every item that cannot be shown
                 * because the container is currently not large enough.
                 */
                this.menu = new Ext.menu.Menu({
                    ownerCt : this.layout.container,
                    listeners: {
                        scope: this,
                        beforeshow: this.beforeMenuShow
                    }
                });

                /**
                 * @private
                 * @property menuTrigger
                 * @type Ext.Button
                 * The expand button which triggers the overflow menu to be shown
                 */
                this.menuTrigger = new Ext.Button({
                    iconCls : 'x-toolbar-more-icon',
                    cls     : 'x-toolbar-more',
                    menu    : this.menu,
                    renderTo: this.afterCt
                });
            }
        },

        /**
         * @private
         */
        destroy: function() {
            Ext.destroy(this.menu, this.menuTrigger);
        }
    });

    Ext.layout.boxOverflow.menu = Ext.layout.boxOverflow.Menu;

    /**
     * @class Ext.layout.boxOverflow.HorizontalMenu
     * @extends Ext.layout.boxOverflow.Menu
     * Description
     */
    Ext.layout.boxOverflow.HorizontalMenu = Ext.extend(Ext.layout.boxOverflow.Menu, {

        constructor: function() {
            Ext.layout.boxOverflow.HorizontalMenu.superclass.constructor.apply(this, arguments);

            var me = this,
                layout = me.layout,
                origFunction = layout.calculateChildBoxes;

            layout.calculateChildBoxes = function(visibleItems, targetSize) {
                var calcs = origFunction.apply(layout, arguments),
                    meta  = calcs.meta,
                    items = me.menuItems;

                //calculate the width of the items currently hidden solely because there is not enough space
                //to display them
                var hiddenWidth = 0;
                for (var index = 0, length = items.length; index < length; index++) {
                    hiddenWidth += items[index].width;
                }

                meta.minimumWidth += hiddenWidth;
                meta.tooNarrow = meta.minimumWidth > targetSize.width;

                return calcs;
            };
        },

        handleOverflow: function(calculations, targetSize) {
            this.showTrigger();

            var newWidth    = targetSize.width - this.afterCt.getWidth(),
                boxes       = calculations.boxes,
                usedWidth   = 0,
                recalculate = false;

            //calculate the width of all visible items and any spare width
            for (var index = 0, length = boxes.length; index < length; index++) {
                usedWidth += boxes[index].width;
            }

            var spareWidth = newWidth - usedWidth,
                showCount  = 0;

            //see if we can re-show any of the hidden components
            for (var index = 0, length = this.menuItems.length; index < length; index++) {
                var hidden = this.menuItems[index],
                    comp   = hidden.component,
                    width  = hidden.width;

                if (width < spareWidth) {
                    comp.show();

                    spareWidth -= width;
                    showCount ++;
                    recalculate = true;
                } else {
                    break;
                }
            }

            if (recalculate) {
                this.menuItems = this.menuItems.slice(showCount);
            } else {
                for (var i = boxes.length - 1; i >= 0; i--) {
                    var item  = boxes[i].component,
                        right = boxes[i].left + boxes[i].width;

                    if (right >= newWidth) {
                        this.menuItems.unshift({
                            component: item,
                            width    : boxes[i].width
                        });

                        item.hide();
                    } else {
                        break;
                    }
                }
            }

            if (this.menuItems.length == 0) {
                this.hideTrigger();
            }

            return {
                targetSize: {
                    height: targetSize.height,
                    width : newWidth
                },
                recalculate: recalculate
            };
        }
    });

    Ext.layout.boxOverflow.menu.hbox = Ext.layout.boxOverflow.HorizontalMenu;