The source code

    /**
     * @class Ext.WindowGroup
     * An object that manages a group of {@link Ext.Window} instances and provides z-order management
     * and window activation behavior.
     * @constructor
     */
    Ext.WindowGroup = function(){
        var list = {};
        var accessList = [];
        var front = null;

        // private
        var sortWindows = function(d1, d2){
            return (!d1._lastAccess || d1._lastAccess < d2._lastAccess) ? -1 : 1;
        };

        // private
        var orderWindows = function(){
            var a = accessList, len = a.length;
            if(len > 0){
                a.sort(sortWindows);
                var seed = a[0].manager.zseed;
                for(var i = 0; i < len; i++){
                    var win = a[i];
                    if(win && !win.hidden){
                        win.setZIndex(seed + (i*10));
                    }
                }
            }
            activateLast();
        };

        // private
        var setActiveWin = function(win){
            if(win != front){
                if(front){
                    front.setActive(false);
                }
                front = win;
                if(win){
                    win.setActive(true);
                }
            }
        };

        // private
        var activateLast = function(){
            for(var i = accessList.length-1; i >=0; --i) {
                if(!accessList[i].hidden){
                    setActiveWin(accessList[i]);
                    return;
                }
            }
            // none to activate
            setActiveWin(null);
        };

        return {
            /**
             * The starting z-index for windows in this WindowGroup (defaults to 9000)
             * @type Number The z-index value
             */
            zseed : 9000,

            /**
             * <p>Registers a {@link Ext.Window Window} with this WindowManager. This should not
             * need to be called under normal circumstances. Windows are automatically registered
             * with a {@link Ext.Window#manager manager} at construction time.</p>
             * <p>Where this may be useful is moving Windows between two WindowManagers. For example,
             * to bring the Ext.MessageBox dialog under the same manager as the Desktop's
             * WindowManager in the desktop sample app:</p><code><pre>
    var msgWin = Ext.MessageBox.getDialog();
    MyDesktop.getDesktop().getManager().register(msgWin);
    </pre></code>
             * @param {Window} win The Window to register.
             */
            register : function(win){
                if(win.manager){
                    win.manager.unregister(win);
                }
                win.manager = this;

                list[win.id] = win;
                accessList.push(win);
                win.on('hide', activateLast);
            },

            /**
             * <p>Unregisters a {@link Ext.Window Window} from this WindowManager. This should not
             * need to be called. Windows are automatically unregistered upon destruction.
             * See {@link #register}.</p>
             * @param {Window} win The Window to unregister.
             */
            unregister : function(win){
                delete win.manager;
                delete list[win.id];
                win.un('hide', activateLast);
                accessList.remove(win);
            },

            /**
             * Gets a registered window by id.
             * @param {String/Object} id The id of the window or a {@link Ext.Window} instance
             * @return {Ext.Window}
             */
            get : function(id){
                return typeof id == "object" ? id : list[id];
            },

            /**
             * Brings the specified window to the front of any other active windows in this WindowGroup.
             * @param {String/Object} win The id of the window or a {@link Ext.Window} instance
             * @return {Boolean} True if the dialog was brought to the front, else false
             * if it was already in front
             */
            bringToFront : function(win){
                win = this.get(win);
                if(win != front){
                    win._lastAccess = new Date().getTime();
                    orderWindows();
                    return true;
                }
                return false;
            },

            /**
             * Sends the specified window to the back of other active windows in this WindowGroup.
             * @param {String/Object} win The id of the window or a {@link Ext.Window} instance
             * @return {Ext.Window} The window
             */
            sendToBack : function(win){
                win = this.get(win);
                win._lastAccess = -(new Date().getTime());
                orderWindows();
                return win;
            },

            /**
             * Hides all windows in this WindowGroup.
             */
            hideAll : function(){
                for(var id in list){
                    if(list[id] && typeof list[id] != "function" && list[id].isVisible()){
                        list[id].hide();
                    }
                }
            },

            /**
             * Gets the currently-active window in this WindowGroup.
             * @return {Ext.Window} The active window
             */
            getActive : function(){
                return front;
            },

            /**
             * Returns zero or more windows in this WindowGroup using the custom search function passed to this method.
             * The function should accept a single {@link Ext.Window} reference as its only argument and should
             * return true if the window matches the search criteria, otherwise it should return false.
             * @param {Function} fn The search function
             * @param {Object} scope (optional) The scope (<code>this</code> reference) in which the function is executed. Defaults to the Window being tested.
             * that gets passed to the function if not specified)
             * @return {Array} An array of zero or more matching windows
             */
            getBy : function(fn, scope){
                var r = [];
                for(var i = accessList.length-1; i >=0; --i) {
                    var win = accessList[i];
                    if(fn.call(scope||win, win) !== false){
                        r.push(win);
                    }
                }
                return r;
            },

            /**
             * Executes the specified function once for every window in this WindowGroup, passing each
             * window as the only parameter. Returning false from the function will stop the iteration.
             * @param {Function} fn The function to execute for each item
             * @param {Object} scope (optional) The scope (<code>this</code> reference) in which the function is executed. Defaults to the current Window in the iteration.
             */
            each : function(fn, scope){
                for(var id in list){
                    if(list[id] && typeof list[id] != "function"){
                        if(fn.call(scope || list[id], list[id]) === false){
                            return;
                        }
                    }
                }
            }
        };
    };

    /**
     * @class Ext.WindowMgr
     * @extends Ext.WindowGroup
     * The default global window group that is available automatically.  To have more than one group of windows
     * with separate z-order stacks, create additional instances of {@link Ext.WindowGroup} as needed.
     * @singleton
     */
    Ext.WindowMgr = new Ext.WindowGroup();