The source code

    /**
     * @class Ext.Element
     */
    Ext.Element.addMethods({
        /**
         * Gets the x,y coordinates specified by the anchor position on the element.
         * @param {String} anchor (optional) The specified anchor position (defaults to "c").  See {@link #alignTo}
         * for details on supported anchor positions.
         * @param {Boolean} local (optional) True to get the local (element top/left-relative) anchor position instead
         * of page coordinates
         * @param {Object} size (optional) An object containing the size to use for calculating anchor position
         * {width: (target width), height: (target height)} (defaults to the element's current size)
         * @return {Array} [x, y] An array containing the element's x and y coordinates
         */
        getAnchorXY : function(anchor, local, s){
            //Passing a different size is useful for pre-calculating anchors,
            //especially for anchored animations that change the el size.
    		anchor = (anchor || "tl").toLowerCase();
            s = s || {};

            var me = this,
            	vp = me.dom == document.body || me.dom == document,
            	w = s.width || vp ? Ext.lib.Dom.getViewWidth() : me.getWidth(),
            	h = s.height || vp ? Ext.lib.Dom.getViewHeight() : me.getHeight(),
            	xy,
            	r = Math.round,
            	o = me.getXY(),
            	scroll = me.getScroll(),
            	extraX = vp ? scroll.left : !local ? o[0] : 0,
            	extraY = vp ? scroll.top : !local ? o[1] : 0,
            	hash = {
    	        	c  : [r(w * 0.5), r(h * 0.5)],
    	        	t  : [r(w * 0.5), 0],
    	        	l  : [0, r(h * 0.5)],
    	        	r  : [w, r(h * 0.5)],
    	        	b  : [r(w * 0.5), h],
    	        	tl : [0, 0],
    	        	bl : [0, h],
    	        	br : [w, h],
    	        	tr : [w, 0]
            	};

            xy = hash[anchor];
            return [xy[0] + extraX, xy[1] + extraY];
        },

        /**
         * Anchors an element to another element and realigns it when the window is resized.
         * @param {Mixed} element The element to align to.
         * @param {String} position The position to align to.
         * @param {Array} offsets (optional) Offset the positioning by [x, y]
         * @param {Boolean/Object} animate (optional) True for the default animation or a standard Element animation config object
         * @param {Boolean/Number} monitorScroll (optional) True to monitor body scroll and reposition. If this parameter
         * is a number, it is used as the buffer delay (defaults to 50ms).
         * @param {Function} callback (optional) The function to call after the animation finishes
         * @return {Ext.Element} this
         */
        anchorTo : function(el, alignment, offsets, animate, monitorScroll, callback){
    	    var me = this,
                dom = me.dom,
                scroll = !Ext.isEmpty(monitorScroll),
                action = function(){
                    Ext.fly(dom).alignTo(el, alignment, offsets, animate);
                    Ext.callback(callback, Ext.fly(dom));
                },
                anchor = this.getAnchor();

            // previous listener anchor, remove it
            this.removeAnchor();
            Ext.apply(anchor, {
                fn: action,
                scroll: scroll
            });

            Ext.EventManager.onWindowResize(action, null);

            if(scroll){
                Ext.EventManager.on(window, 'scroll', action, null,
                    {buffer: !isNaN(monitorScroll) ? monitorScroll : 50});
            }
            action.call(me); // align immediately
            return me;
        },

        /**
         * Remove any anchor to this element. See {@link #anchorTo}.
         * @return {Ext.Element} this
         */
        removeAnchor : function(){
            var me = this,
                anchor = this.getAnchor();

            if(anchor && anchor.fn){
                Ext.EventManager.removeResizeListener(anchor.fn);
                if(anchor.scroll){
                    Ext.EventManager.un(window, 'scroll', anchor.fn);
                }
                delete anchor.fn;
            }
            return me;
        },

        // private
        getAnchor : function(){
            var data = Ext.Element.data,
                dom = this.dom;
                if (!dom) {
                    return;
                }
                var anchor = data(dom, '_anchor');

            if(!anchor){
                anchor = data(dom, '_anchor', {});
            }
            return anchor;
        },

        /**
         * Gets the x,y coordinates to align this element with another element. See {@link #alignTo} for more info on the
         * supported position values.
         * @param {Mixed} element The element to align to.
         * @param {String} position (optional, defaults to "tl-bl?") The position to align to.
         * @param {Array} offsets (optional) Offset the positioning by [x, y]
         * @return {Array} [x, y]
         */
        getAlignToXY : function(el, p, o){
            el = Ext.get(el);

            if(!el || !el.dom){
                throw "Element.alignToXY with an element that doesn't exist";
            }

            o = o || [0,0];
            p = (!p || p == "?" ? "tl-bl?" : (!(/-/).test(p) && p !== "" ? "tl-" + p : p || "tl-bl")).toLowerCase();

            var me = this,
            	d = me.dom,
            	a1,
            	a2,
            	x,
            	y,
            	//constrain the aligned el to viewport if necessary
            	w,
            	h,
            	r,
            	dw = Ext.lib.Dom.getViewWidth() -10, // 10px of margin for ie
            	dh = Ext.lib.Dom.getViewHeight()-10, // 10px of margin for ie
            	p1y,
            	p1x,
            	p2y,
            	p2x,
            	swapY,
            	swapX,
            	doc = document,
            	docElement = doc.documentElement,
            	docBody = doc.body,
            	scrollX = (docElement.scrollLeft || docBody.scrollLeft || 0)+5,
            	scrollY = (docElement.scrollTop || docBody.scrollTop || 0)+5,
            	c = false, //constrain to viewport
            	p1 = "",
            	p2 = "",
            	m = p.match(/^([a-z]+)-([a-z]+)(\?)?$/);

            if(!m){
               throw "Element.alignTo with an invalid alignment " + p;
            }

            p1 = m[1];
            p2 = m[2];
            c = !!m[3];

            //Subtract the aligned el's internal xy from the target's offset xy
            //plus custom offset to get the aligned el's new offset xy
            a1 = me.getAnchorXY(p1, true);
            a2 = el.getAnchorXY(p2, false);

            x = a2[0] - a1[0] + o[0];
            y = a2[1] - a1[1] + o[1];

            if(c){
    	       w = me.getWidth();
               h = me.getHeight();
               r = el.getRegion();
               //If we are at a viewport boundary and the aligned el is anchored on a target border that is
               //perpendicular to the vp border, allow the aligned el to slide on that border,
               //otherwise swap the aligned el to the opposite border of the target.
               p1y = p1.charAt(0);
               p1x = p1.charAt(p1.length-1);
               p2y = p2.charAt(0);
               p2x = p2.charAt(p2.length-1);
               swapY = ((p1y=="t" && p2y=="b") || (p1y=="b" && p2y=="t"));
               swapX = ((p1x=="r" && p2x=="l") || (p1x=="l" && p2x=="r"));

               if (x + w > dw + scrollX) {
                    x = swapX ? r.left-w : dw+scrollX-w;
               }
               if (x < scrollX) {
                   x = swapX ? r.right : scrollX;
               }
               if (y + h > dh + scrollY) {
                    y = swapY ? r.top-h : dh+scrollY-h;
                }
               if (y < scrollY){
                   y = swapY ? r.bottom : scrollY;
               }
            }
            return [x,y];
        },

        /**
         * Aligns this element with another element relative to the specified anchor points. If the other element is the
         * document it aligns it to the viewport.
         * The position parameter is optional, and can be specified in any one of the following formats:
         * <ul>
         *   <li><b>Blank</b>: Defaults to aligning the element's top-left corner to the target's bottom-left corner ("tl-bl").</li>
         *   <li><b>One anchor (deprecated)</b>: The passed anchor position is used as the target element's anchor point.
         *       The element being aligned will position its top-left corner (tl) to that point.  <i>This method has been
         *       deprecated in favor of the newer two anchor syntax below</i>.</li>
         *   <li><b>Two anchors</b>: If two values from the table below are passed separated by a dash, the first value is used as the
         *       element's anchor point, and the second value is used as the target's anchor point.</li>
         * </ul>
         * In addition to the anchor points, the position parameter also supports the "?" character.  If "?" is passed at the end of
         * the position string, the element will attempt to align as specified, but the position will be adjusted to constrain to
         * the viewport if necessary.  Note that the element being aligned might be swapped to align to a different position than
         * that specified in order to enforce the viewport constraints.
         * Following are all of the supported anchor positions:
    <pre>
    Value  Description
    -----  -----------------------------
    tl     The top left corner (default)
    t      The center of the top edge
    tr     The top right corner
    l      The center of the left edge
    c      In the center of the element
    r      The center of the right edge
    bl     The bottom left corner
    b      The center of the bottom edge
    br     The bottom right corner
    </pre>
    Example Usage:
    <pre><code>
    // align el to other-el using the default positioning ("tl-bl", non-constrained)
    el.alignTo("other-el");

    // align the top left corner of el with the top right corner of other-el (constrained to viewport)
    el.alignTo("other-el", "tr?");

    // align the bottom right corner of el with the center left edge of other-el
    el.alignTo("other-el", "br-l?");

    // align the center of el with the bottom left corner of other-el and
    // adjust the x position by -6 pixels (and the y position by 0)
    el.alignTo("other-el", "c-bl", [-6, 0]);
    </code></pre>
         * @param {Mixed} element The element to align to.
         * @param {String} position (optional, defaults to "tl-bl?") The position to align to.
         * @param {Array} offsets (optional) Offset the positioning by [x, y]
         * @param {Boolean/Object} animate (optional) true for the default animation or a standard Element animation config object
         * @return {Ext.Element} this
         */
        alignTo : function(element, position, offsets, animate){
    	    var me = this;
            return me.setXY(me.getAlignToXY(element, position, offsets),
              		        me.preanim && !!animate ? me.preanim(arguments, 3) : false);
        },

        // private ==>  used outside of core
        adjustForConstraints : function(xy, parent, offsets){
            return this.getConstrainToXY(parent || document, false, offsets, xy) ||  xy;
        },

        // private ==>  used outside of core
        getConstrainToXY : function(el, local, offsets, proposedXY){
    	    var os = {top:0, left:0, bottom:0, right: 0};

            return function(el, local, offsets, proposedXY){
                el = Ext.get(el);
                offsets = offsets ? Ext.applyIf(offsets, os) : os;

                var vw, vh, vx = 0, vy = 0;
                if(el.dom == document.body || el.dom == document){
                    vw =Ext.lib.Dom.getViewWidth();
                    vh = Ext.lib.Dom.getViewHeight();
                }else{
                    vw = el.dom.clientWidth;
                    vh = el.dom.clientHeight;
                    if(!local){
                        var vxy = el.getXY();
                        vx = vxy[0];
                        vy = vxy[1];
                    }
                }

                var s = el.getScroll();

                vx += offsets.left + s.left;
                vy += offsets.top + s.top;

                vw -= offsets.right;
                vh -= offsets.bottom;

                var vr = vx + vw,
                    vb = vy + vh,
                    xy = proposedXY || (!local ? this.getXY() : [this.getLeft(true), this.getTop(true)]),
                    x = xy[0], y = xy[1],
                    offset = this.getConstrainOffset(),
                    w = this.dom.offsetWidth + offset,
                    h = this.dom.offsetHeight + offset;

                // only move it if it needs it
                var moved = false;

                // first validate right/bottom
                if((x + w) > vr){
                    x = vr - w;
                    moved = true;
                }
                if((y + h) > vb){
                    y = vb - h;
                    moved = true;
                }
                // then make sure top/left isn't negative
                if(x < vx){
                    x = vx;
                    moved = true;
                }
                if(y < vy){
                    y = vy;
                    moved = true;
                }
                return moved ? [x, y] : false;
            };
        }(),

    //         el = Ext.get(el);
    //         offsets = Ext.applyIf(offsets || {}, {top : 0, left : 0, bottom : 0, right : 0});

    //         var	me = this,
    //         	doc = document,
    //         	s = el.getScroll(),
    //         	vxy = el.getXY(),
    //         	vx = offsets.left + s.left,
    //         	vy = offsets.top + s.top,
    //         	vw = -offsets.right,
    //         	vh = -offsets.bottom,
    //         	vr,
    //         	vb,
    //         	xy = proposedXY || (!local ? me.getXY() : [me.getLeft(true), me.getTop(true)]),
    //         	x = xy[0],
    //         	y = xy[1],
    //         	w = me.dom.offsetWidth, h = me.dom.offsetHeight,
    //         	moved = false; // only move it if it needs it
    //
    //
    //         if(el.dom == doc.body || el.dom == doc){
    //             vw += Ext.lib.Dom.getViewWidth();
    //             vh += Ext.lib.Dom.getViewHeight();
    //         }else{
    //             vw += el.dom.clientWidth;
    //             vh += el.dom.clientHeight;
    //             if(!local){
    //                 vx += vxy[0];
    //                 vy += vxy[1];
    //             }
    //         }

    //         // first validate right/bottom
    //         if(x + w > vx + vw){
    //             x = vx + vw - w;
    //             moved = true;
    //         }
    //         if(y + h > vy + vh){
    //             y = vy + vh - h;
    //             moved = true;
    //         }
    //         // then make sure top/left isn't negative
    //         if(x < vx){
    //             x = vx;
    //             moved = true;
    //         }
    //         if(y < vy){
    //             y = vy;
    //             moved = true;
    //         }
    //         return moved ? [x, y] : false;
    //    },

        // private, used internally
        getConstrainOffset : function(){
            return 0;
        },

        /**
        * Calculates the x, y to center this element on the screen
        * @return {Array} The x, y values [x, y]
        */
        getCenterXY : function(){
            return this.getAlignToXY(document, 'c-c');
        },

        /**
        * Centers the Element in either the viewport, or another Element.
        * @param {Mixed} centerIn (optional) The element in which to center the element.
        */
        center : function(centerIn){
            return this.alignTo(centerIn || document, 'c-c');
        }
    });