The source code

    /**
     * @class Ext.tree.TreeEditor
     * @extends Ext.Editor
     * Provides editor functionality for inline tree node editing.  Any valid {@link Ext.form.Field} subclass can be used
     * as the editor field.
     * @constructor
     * @param {TreePanel} tree
     * @param {Object} fieldConfig (optional) Either a prebuilt {@link Ext.form.Field} instance or a Field config object
     * that will be applied to the default field instance (defaults to a {@link Ext.form.TextField}).
     * @param {Object} config (optional) A TreeEditor config object
     */
    Ext.tree.TreeEditor = function(tree, fc, config){
        fc = fc || {};
        var field = fc.events ? fc : new Ext.form.TextField(fc);

        Ext.tree.TreeEditor.superclass.constructor.call(this, field, config);

        this.tree = tree;

        if(!tree.rendered){
            tree.on('render', this.initEditor, this);
        }else{
            this.initEditor(tree);
        }
    };

    Ext.extend(Ext.tree.TreeEditor, Ext.Editor, {
        /**
         * @cfg {String} alignment
         * The position to align to (see {@link Ext.Element#alignTo} for more details, defaults to "l-l").
         */
        alignment: "l-l",
        // inherit
        autoSize: false,
        /**
         * @cfg {Boolean} hideEl
         * True to hide the bound element while the editor is displayed (defaults to false)
         */
        hideEl : false,
        /**
         * @cfg {String} cls
         * CSS class to apply to the editor (defaults to "x-small-editor x-tree-editor")
         */
        cls: "x-small-editor x-tree-editor",
        /**
         * @cfg {Boolean} shim
         * True to shim the editor if selects/iframes could be displayed beneath it (defaults to false)
         */
        shim:false,
        // inherit
        shadow:"frame",
        /**
         * @cfg {Number} maxWidth
         * The maximum width in pixels of the editor field (defaults to 250).  Note that if the maxWidth would exceed
         * the containing tree element's size, it will be automatically limited for you to the container width, taking
         * scroll and client offsets into account prior to each edit.
         */
        maxWidth: 250,
        /**
         * @cfg {Number} editDelay The number of milliseconds between clicks to register a double-click that will trigger
         * editing on the current node (defaults to 350).  If two clicks occur on the same node within this time span,
         * the editor for the node will display, otherwise it will be processed as a regular click.
         */
        editDelay : 350,

        initEditor : function(tree){
            tree.on({
                scope      : this,
                beforeclick: this.beforeNodeClick,
                dblclick   : this.onNodeDblClick
            });

            this.on({
                scope          : this,
                complete       : this.updateNode,
                beforestartedit: this.fitToTree,
                specialkey     : this.onSpecialKey
            });

            this.on('startedit', this.bindScroll, this, {delay:10});
        },

        // private
        fitToTree : function(ed, el){
            var td = this.tree.getTreeEl().dom, nd = el.dom;
            if(td.scrollLeft >  nd.offsetLeft){ // ensure the node left point is visible
                td.scrollLeft = nd.offsetLeft;
            }
            var w = Math.min(
                    this.maxWidth,
                    (td.clientWidth > 20 ? td.clientWidth : td.offsetWidth) - Math.max(0, nd.offsetLeft-td.scrollLeft) - /*cushion*/5);
            this.setSize(w, '');
        },

        /**
         * Edit the text of the passed {@link Ext.tree.TreeNode TreeNode}.
         * @param node {Ext.tree.TreeNode} The TreeNode to edit. The TreeNode must be {@link Ext.tree.TreeNode#editable editable}.
         */
        triggerEdit : function(node, defer){
            this.completeEdit();
    		if(node.attributes.editable !== false){
               /**
                * The {@link Ext.tree.TreeNode TreeNode} this editor is bound to. Read-only.
                * @type Ext.tree.TreeNode
                * @property editNode
                */
    			this.editNode = node;
                if(this.tree.autoScroll){
                    Ext.fly(node.ui.getEl()).scrollIntoView(this.tree.body);
                }
                var value = node.text || '';
                if (!Ext.isGecko && Ext.isEmpty(node.text)){
                    node.setText('&#160;');
                }
                this.autoEditTimer = this.startEdit.defer(this.editDelay, this, [node.ui.textNode, value]);
                return false;
            }
        },

        // private
        bindScroll : function(){
            this.tree.getTreeEl().on('scroll', this.cancelEdit, this);
        },

        // private
        beforeNodeClick : function(node, e){
            clearTimeout(this.autoEditTimer);
            if(this.tree.getSelectionModel().isSelected(node)){
                e.stopEvent();
                return this.triggerEdit(node);
            }
        },

        onNodeDblClick : function(node, e){
            clearTimeout(this.autoEditTimer);
        },

        // private
        updateNode : function(ed, value){
            this.tree.getTreeEl().un('scroll', this.cancelEdit, this);
            this.editNode.setText(value);
        },

        // private
        onHide : function(){
            Ext.tree.TreeEditor.superclass.onHide.call(this);
            if(this.editNode){
                this.editNode.ui.focus.defer(50, this.editNode.ui);
            }
        },

        // private
        onSpecialKey : function(field, e){
            var k = e.getKey();
            if(k == e.ESC){
                e.stopEvent();
                this.cancelEdit();
            }else if(k == e.ENTER && !e.hasModifier()){
                e.stopEvent();
                this.completeEdit();
            }
        },

        onDestroy : function(){
            clearTimeout(this.autoEditTimer);
            Ext.tree.TreeEditor.superclass.onDestroy.call(this);
            var tree = this.tree;
            tree.un('beforeclick', this.beforeNodeClick, this);
            tree.un('dblclick', this.onNodeDblClick, this);
        }
    });