The source code

    /**
     * @class Ext.ToolTip
     * @extends Ext.Tip
     * A standard tooltip implementation for providing additional information when hovering over a target element.
     * @xtype tooltip
     * @constructor
     * Create a new Tooltip
     * @param {Object} config The configuration options
     */
    Ext.ToolTip = Ext.extend(Ext.Tip, {
        /**
         * When a Tooltip is configured with the <code>{@link #delegate}</code>
         * option to cause selected child elements of the <code>{@link #target}</code>
         * Element to each trigger a seperate show event, this property is set to
         * the DOM element which triggered the show.
         * @type DOMElement
         * @property triggerElement
         */
        /**
         * @cfg {Mixed} target The target HTMLElement, Ext.Element or id to monitor
         * for mouseover events to trigger showing this ToolTip.
         */
        /**
         * @cfg {Boolean} autoHide True to automatically hide the tooltip after the
         * mouse exits the target element or after the <code>{@link #dismissDelay}</code>
         * has expired if set (defaults to true).  If <code>{@link closable} = true</code>
         * a close tool button will be rendered into the tooltip header.
         */
        /**
         * @cfg {Number} showDelay Delay in milliseconds before the tooltip displays
         * after the mouse enters the target element (defaults to 500)
         */
        showDelay : 500,
        /**
         * @cfg {Number} hideDelay Delay in milliseconds after the mouse exits the
         * target element but before the tooltip actually hides (defaults to 200).
         * Set to 0 for the tooltip to hide immediately.
         */
        hideDelay : 200,
        /**
         * @cfg {Number} dismissDelay Delay in milliseconds before the tooltip
         * automatically hides (defaults to 5000). To disable automatic hiding, set
         * dismissDelay = 0.
         */
        dismissDelay : 5000,
        /**
         * @cfg {Array} mouseOffset An XY offset from the mouse position where the
         * tooltip should be shown (defaults to [15,18]).
         */
        /**
         * @cfg {Boolean} trackMouse True to have the tooltip follow the mouse as it
         * moves over the target element (defaults to false).
         */
        trackMouse : false,
        /**
         * @cfg {Boolean} anchorToTarget True to anchor the tooltip to the target
         * element, false to anchor it relative to the mouse coordinates (defaults
         * to true).  When <code>anchorToTarget</code> is true, use
         * <code>{@link #defaultAlign}</code> to control tooltip alignment to the
         * target element.  When <code>anchorToTarget</code> is false, use
         * <code>{@link #anchorPosition}</code> instead to control alignment.
         */
        anchorToTarget : true,
        /**
         * @cfg {Number} anchorOffset A numeric pixel value used to offset the
         * default position of the anchor arrow (defaults to 0).  When the anchor
         * position is on the top or bottom of the tooltip, <code>anchorOffset</code>
         * will be used as a horizontal offset.  Likewise, when the anchor position
         * is on the left or right side, <code>anchorOffset</code> will be used as
         * a vertical offset.
         */
        anchorOffset : 0,
        /**
         * @cfg {String} delegate <p>Optional. A {@link Ext.DomQuery DomQuery}
         * selector which allows selection of individual elements within the
         * <code>{@link #target}</code> element to trigger showing and hiding the
         * ToolTip as the mouse moves within the target.</p>
         * <p>When specified, the child element of the target which caused a show
         * event is placed into the <code>{@link #triggerElement}</code> property
         * before the ToolTip is shown.</p>
         * <p>This may be useful when a Component has regular, repeating elements
         * in it, each of which need a Tooltip which contains information specific
         * to that element. For example:</p><pre><code>
    var myGrid = new Ext.grid.gridPanel(gridConfig);
    myGrid.on('render', function(grid) {
        var store = grid.getStore();  // Capture the Store.
        var view = grid.getView();    // Capture the GridView.
        myGrid.tip = new Ext.ToolTip({
            target: view.mainBody,    // The overall target element.
            delegate: '.x-grid3-row', // Each grid row causes its own seperate show and hide.
            trackMouse: true,         // Moving within the row should not hide the tip.
            renderTo: document.body,  // Render immediately so that tip.body can be
                                      //  referenced prior to the first show.
            listeners: {              // Change content dynamically depending on which element
                                      //  triggered the show.
                beforeshow: function updateTipBody(tip) {
                    var rowIndex = view.findRowIndex(tip.triggerElement);
                    tip.body.dom.innerHTML = 'Over Record ID ' + store.getAt(rowIndex).id;
                }
            }
        });
    });
         *</code></pre>
         */

        // private
        targetCounter : 0,

        constrainPosition : false,

        // private
        initComponent : function(){
            Ext.ToolTip.superclass.initComponent.call(this);
            this.lastActive = new Date();
            this.initTarget(this.target);
            this.origAnchor = this.anchor;
        },

        // private
        onRender : function(ct, position){
            Ext.ToolTip.superclass.onRender.call(this, ct, position);
            this.anchorCls = 'x-tip-anchor-' + this.getAnchorPosition();
            this.anchorEl = this.el.createChild({
                cls: 'x-tip-anchor ' + this.anchorCls
            });
        },

        // private
        afterRender : function(){
            Ext.ToolTip.superclass.afterRender.call(this);
            this.anchorEl.setStyle('z-index', this.el.getZIndex() + 1).setVisibilityMode(Ext.Element.DISPLAY);
        },

        /**
         * Binds this ToolTip to the specified element. The tooltip will be displayed when the mouse moves over the element.
         * @param {Mixed} t The Element, HtmlElement, or ID of an element to bind to
         */
        initTarget : function(target){
            var t;
            if((t = Ext.get(target))){
                if(this.target){
                    var tg = Ext.get(this.target);
                    this.mun(tg, 'mouseover', this.onTargetOver, this);
                    this.mun(tg, 'mouseout', this.onTargetOut, this);
                    this.mun(tg, 'mousemove', this.onMouseMove, this);
                }
                this.mon(t, {
                    mouseover: this.onTargetOver,
                    mouseout: this.onTargetOut,
                    mousemove: this.onMouseMove,
                    scope: this
                });
                this.target = t;
            }
            if(this.anchor){
                this.anchorTarget = this.target;
            }
        },

        // private
        onMouseMove : function(e){
            var t = this.delegate ? e.getTarget(this.delegate) : this.triggerElement = true;
            if (t) {
                this.targetXY = e.getXY();
                if (t === this.triggerElement) {
                    if(!this.hidden && this.trackMouse){
                        this.setPagePosition(this.getTargetXY());
                    }
                } else {
                    this.hide();
                    this.lastActive = new Date(0);
                    this.onTargetOver(e);
                }
            } else if (!this.closable && this.isVisible()) {
                this.hide();
            }
        },

        // private
        getTargetXY : function(){
            if(this.delegate){
                this.anchorTarget = this.triggerElement;
            }
            if(this.anchor){
                this.targetCounter++;
                var offsets = this.getOffsets(),
                    xy = (this.anchorToTarget && !this.trackMouse) ? this.el.getAlignToXY(this.anchorTarget, this.getAnchorAlign()) : this.targetXY,
                    dw = Ext.lib.Dom.getViewWidth() - 5,
                    dh = Ext.lib.Dom.getViewHeight() - 5,
                    de = document.documentElement,
                    bd = document.body,
                    scrollX = (de.scrollLeft || bd.scrollLeft || 0) + 5,
                    scrollY = (de.scrollTop || bd.scrollTop || 0) + 5,
                    axy = [xy[0] + offsets[0], xy[1] + offsets[1]],
                    sz = this.getSize();

                this.anchorEl.removeClass(this.anchorCls);

                if(this.targetCounter < 2){
                    if(axy[0] < scrollX){
                        if(this.anchorToTarget){
                            this.defaultAlign = 'l-r';
                            if(this.mouseOffset){this.mouseOffset[0] *= -1;}
                        }
                        this.anchor = 'left';
                        return this.getTargetXY();
                    }
                    if(axy[0]+sz.width > dw){
                        if(this.anchorToTarget){
                            this.defaultAlign = 'r-l';
                            if(this.mouseOffset){this.mouseOffset[0] *= -1;}
                        }
                        this.anchor = 'right';
                        return this.getTargetXY();
                    }
                    if(axy[1] < scrollY){
                        if(this.anchorToTarget){
                            this.defaultAlign = 't-b';
                            if(this.mouseOffset){this.mouseOffset[1] *= -1;}
                        }
                        this.anchor = 'top';
                        return this.getTargetXY();
                    }
                    if(axy[1]+sz.height > dh){
                        if(this.anchorToTarget){
                            this.defaultAlign = 'b-t';
                            if(this.mouseOffset){this.mouseOffset[1] *= -1;}
                        }
                        this.anchor = 'bottom';
                        return this.getTargetXY();
                    }
                }

                this.anchorCls = 'x-tip-anchor-'+this.getAnchorPosition();
                this.anchorEl.addClass(this.anchorCls);
                this.targetCounter = 0;
                return axy;
            }else{
                var mouseOffset = this.getMouseOffset();
                return [this.targetXY[0]+mouseOffset[0], this.targetXY[1]+mouseOffset[1]];
            }
        },

        getMouseOffset : function(){
            var offset = this.anchor ? [0,0] : [15,18];
            if(this.mouseOffset){
                offset[0] += this.mouseOffset[0];
                offset[1] += this.mouseOffset[1];
            }
            return offset;
        },

        // private
        getAnchorPosition : function(){
            if(this.anchor){
                this.tipAnchor = this.anchor.charAt(0);
            }else{
                var m = this.defaultAlign.match(/^([a-z]+)-([a-z]+)(\?)?$/);
                if(!m){
                   throw 'AnchorTip.defaultAlign is invalid';
                }
                this.tipAnchor = m[1].charAt(0);
            }

            switch(this.tipAnchor){
                case 't': return 'top';
                case 'b': return 'bottom';
                case 'r': return 'right';
            }
            return 'left';
        },

        // private
        getAnchorAlign : function(){
            switch(this.anchor){
                case 'top'  : return 'tl-bl';
                case 'left' : return 'tl-tr';
                case 'right': return 'tr-tl';
                default     : return 'bl-tl';
            }
        },

        // private
        getOffsets : function(){
            var offsets,
                ap = this.getAnchorPosition().charAt(0);
            if(this.anchorToTarget && !this.trackMouse){
                switch(ap){
                    case 't':
                        offsets = [0, 9];
                        break;
                    case 'b':
                        offsets = [0, -13];
                        break;
                    case 'r':
                        offsets = [-13, 0];
                        break;
                    default:
                        offsets = [9, 0];
                        break;
                }
            }else{
                switch(ap){
                    case 't':
                        offsets = [-15-this.anchorOffset, 30];
                        break;
                    case 'b':
                        offsets = [-19-this.anchorOffset, -13-this.el.dom.offsetHeight];
                        break;
                    case 'r':
                        offsets = [-15-this.el.dom.offsetWidth, -13-this.anchorOffset];
                        break;
                    default:
                        offsets = [25, -13-this.anchorOffset];
                        break;
                }
            }
            var mouseOffset = this.getMouseOffset();
            offsets[0] += mouseOffset[0];
            offsets[1] += mouseOffset[1];

            return offsets;
        },

        // private
        onTargetOver : function(e){
            if(this.disabled || e.within(this.target.dom, true)){
                return;
            }
            var t = e.getTarget(this.delegate);
            if (t) {
                this.triggerElement = t;
                this.clearTimer('hide');
                this.targetXY = e.getXY();
                this.delayShow();
            }
        },

        // private
        delayShow : function(){
            if(this.hidden && !this.showTimer){
                if(this.lastActive.getElapsed() < this.quickShowInterval){
                    this.show();
                }else{
                    this.showTimer = this.show.defer(this.showDelay, this);
                }
            }else if(!this.hidden && this.autoHide !== false){
                this.show();
            }
        },

        // private
        onTargetOut : function(e){
            if(this.disabled || e.within(this.target.dom, true)){
                return;
            }
            this.clearTimer('show');
            if(this.autoHide !== false){
                this.delayHide();
            }
        },

        // private
        delayHide : function(){
            if(!this.hidden && !this.hideTimer){
                this.hideTimer = this.hide.defer(this.hideDelay, this);
            }
        },

        /**
         * Hides this tooltip if visible.
         */
        hide: function(){
            this.clearTimer('dismiss');
            this.lastActive = new Date();
            if(this.anchorEl){
                this.anchorEl.hide();
            }
            Ext.ToolTip.superclass.hide.call(this);
            delete this.triggerElement;
        },

        /**
         * Shows this tooltip at the current event target XY position.
         */
        show : function(){
            if(this.anchor){
                // pre-show it off screen so that the el will have dimensions
                // for positioning calcs when getting xy next
                this.showAt([-1000,-1000]);
                this.origConstrainPosition = this.constrainPosition;
                this.constrainPosition = false;
                this.anchor = this.origAnchor;
            }
            this.showAt(this.getTargetXY());

            if(this.anchor){
                this.anchorEl.show();
                this.syncAnchor();
                this.constrainPosition = this.origConstrainPosition;
            }else{
                this.anchorEl.hide();
            }
        },

        // inherit docs
        showAt : function(xy){
            this.lastActive = new Date();
            this.clearTimers();
            Ext.ToolTip.superclass.showAt.call(this, xy);
            if(this.dismissDelay && this.autoHide !== false){
                this.dismissTimer = this.hide.defer(this.dismissDelay, this);
            }
            if(this.anchor && !this.anchorEl.isVisible()){
                this.syncAnchor();
                this.anchorEl.show();
            }else{
                this.anchorEl.hide();
            }
        },

        // private
        syncAnchor : function(){
            var anchorPos, targetPos, offset;
            switch(this.tipAnchor.charAt(0)){
                case 't':
                    anchorPos = 'b';
                    targetPos = 'tl';
                    offset = [20+this.anchorOffset, 2];
                    break;
                case 'r':
                    anchorPos = 'l';
                    targetPos = 'tr';
                    offset = [-2, 11+this.anchorOffset];
                    break;
                case 'b':
                    anchorPos = 't';
                    targetPos = 'bl';
                    offset = [20+this.anchorOffset, -2];
                    break;
                default:
                    anchorPos = 'r';
                    targetPos = 'tl';
                    offset = [2, 11+this.anchorOffset];
                    break;
            }
            this.anchorEl.alignTo(this.el, anchorPos+'-'+targetPos, offset);
        },

        // private
        setPagePosition : function(x, y){
            Ext.ToolTip.superclass.setPagePosition.call(this, x, y);
            if(this.anchor){
                this.syncAnchor();
            }
        },

        // private
        clearTimer : function(name){
            name = name + 'Timer';
            clearTimeout(this[name]);
            delete this[name];
        },

        // private
        clearTimers : function(){
            this.clearTimer('show');
            this.clearTimer('dismiss');
            this.clearTimer('hide');
        },

        // private
        onShow : function(){
            Ext.ToolTip.superclass.onShow.call(this);
            Ext.getDoc().on('mousedown', this.onDocMouseDown, this);
        },

        // private
        onHide : function(){
            Ext.ToolTip.superclass.onHide.call(this);
            Ext.getDoc().un('mousedown', this.onDocMouseDown, this);
        },

        // private
        onDocMouseDown : function(e){
            if(this.autoHide !== true && !this.closable && !e.within(this.el.dom)){
                this.disable();
                this.doEnable.defer(100, this);
            }
        },

        // private
        doEnable : function(){
            if(!this.isDestroyed){
                this.enable();
            }
        },

        // private
        onDisable : function(){
            this.clearTimers();
            this.hide();
        },

        // private
        adjustPosition : function(x, y){
            if(this.constrainPosition){
                var ay = this.targetXY[1], h = this.getSize().height;
                if(y <= ay && (y+h) >= ay){
                    y = ay-h-5;
                }
            }
            return {x : x, y: y};
        },

        beforeDestroy : function(){
            this.clearTimers();
            Ext.destroy(this.anchorEl);
            delete this.anchorEl;
            delete this.target;
            delete this.anchorTarget;
            delete this.triggerElement;
            Ext.ToolTip.superclass.beforeDestroy.call(this);
        },

        // private
        onDestroy : function(){
            Ext.getDoc().un('mousedown', this.onDocMouseDown, this);
            Ext.ToolTip.superclass.onDestroy.call(this);
        }
    });

    Ext.reg('tooltip', Ext.ToolTip);