The source code

    /*!
     * Ext JS Library 3.2.1
     * Copyright(c) 2006-2010 Ext JS, Inc.
     * licensing@extjs.com
     * http://www.extjs.com/license
     */
    /**
     * @class Ext.util.TextMetrics
     * Provides precise pixel measurements for blocks of text so that you can determine exactly how high and
     * wide, in pixels, a given block of text will be. Note that when measuring text, it should be plain text and
     * should not contain any HTML, otherwise it may not be measured correctly.
     * @singleton
     */
    Ext.util.TextMetrics = function(){
        var shared;
        return {
            /**
             * Measures the size of the specified text
             * @param {String/HTMLElement} el The element, dom node or id from which to copy existing CSS styles
             * that can affect the size of the rendered text
             * @param {String} text The text to measure
             * @param {Number} fixedWidth (optional) If the text will be multiline, you have to set a fixed width
             * in order to accurately measure the text height
             * @return {Object} An object containing the text's size {width: (width), height: (height)}
             */
            measure : function(el, text, fixedWidth){
                if(!shared){
                    shared = Ext.util.TextMetrics.Instance(el, fixedWidth);
                }
                shared.bind(el);
                shared.setFixedWidth(fixedWidth || 'auto');
                return shared.getSize(text);
            },

            /**
             * Return a unique TextMetrics instance that can be bound directly to an element and reused.  This reduces
             * the overhead of multiple calls to initialize the style properties on each measurement.
             * @param {String/HTMLElement} el The element, dom node or id that the instance will be bound to
             * @param {Number} fixedWidth (optional) If the text will be multiline, you have to set a fixed width
             * in order to accurately measure the text height
             * @return {Ext.util.TextMetrics.Instance} instance The new instance
             */
            createInstance : function(el, fixedWidth){
                return Ext.util.TextMetrics.Instance(el, fixedWidth);
            }
        };
    }();

    Ext.util.TextMetrics.Instance = function(bindTo, fixedWidth){
        var ml = new Ext.Element(document.createElement('div'));
        document.body.appendChild(ml.dom);
        ml.position('absolute');
        ml.setLeftTop(-1000, -1000);
        ml.hide();

        if(fixedWidth){
            ml.setWidth(fixedWidth);
        }

        var instance = {
            /**
             * Only available on the instance returned from {@link #createInstance}, not on the singleton.
             * Returns the size of the specified text based on the internal element's style and width properties
             * @param {String} text The text to measure
             * @return {Object} An object containing the text's size {width: (width), height: (height)}
             */
            getSize : function(text){
                ml.update(text);
                var s = ml.getSize();
                ml.update('');
                return s;
            },

            /**
             * Only available on the instance returned from {@link #createInstance}, not on the singleton.
             * Binds this TextMetrics instance to an element from which to copy existing CSS styles
             * that can affect the size of the rendered text
             * @param {String/HTMLElement} el The element, dom node or id
             */
            bind : function(el){
                ml.setStyle(
                    Ext.fly(el).getStyles('font-size','font-style', 'font-weight', 'font-family','line-height', 'text-transform', 'letter-spacing')
                );
            },

            /**
             * Only available on the instance returned from {@link #createInstance}, not on the singleton.
             * Sets a fixed width on the internal measurement element.  If the text will be multiline, you have
             * to set a fixed width in order to accurately measure the text height.
             * @param {Number} width The width to set on the element
             */
            setFixedWidth : function(width){
                ml.setWidth(width);
            },

            /**
             * Only available on the instance returned from {@link #createInstance}, not on the singleton.
             * Returns the measured width of the specified text
             * @param {String} text The text to measure
             * @return {Number} width The width in pixels
             */
            getWidth : function(text){
                ml.dom.style.width = 'auto';
                return this.getSize(text).width;
            },

            /**
             * Only available on the instance returned from {@link #createInstance}, not on the singleton.
             * Returns the measured height of the specified text.  For multiline text, be sure to call
             * {@link #setFixedWidth} if necessary.
             * @param {String} text The text to measure
             * @return {Number} height The height in pixels
             */
            getHeight : function(text){
                return this.getSize(text).height;
            }
        };

        instance.bind(bindTo);

        return instance;
    };

    Ext.Element.addMethods({
        /**
         * Returns the width in pixels of the passed text, or the width of the text in this Element.
         * @param {String} text The text to measure. Defaults to the innerHTML of the element.
         * @param {Number} min (Optional) The minumum value to return.
         * @param {Number} max (Optional) The maximum value to return.
         * @return {Number} The text width in pixels.
         * @member Ext.Element getTextWidth
         */
        getTextWidth : function(text, min, max){
            return (Ext.util.TextMetrics.measure(this.dom, Ext.value(text, this.dom.innerHTML, true)).width).constrain(min || 0, max || 1000000);
        }
    });