The source code

    /**
     * @class Ext.Shadow
     * Simple class that can provide a shadow effect for any element.  Note that the element MUST be absolutely positioned,
     * and the shadow does not provide any shimming.  This should be used only in simple cases -- for more advanced
     * functionality that can also provide the same shadow effect, see the {@link Ext.Layer} class.
     * @constructor
     * Create a new Shadow
     * @param {Object} config The config object
     */
    Ext.Shadow = function(config) {
        Ext.apply(this, config);
        if (typeof this.mode != "string") {
            this.mode = this.defaultMode;
        }
        var o = this.offset,
            a = {
                h: 0
            },
            rad = Math.floor(this.offset / 2);
        switch (this.mode.toLowerCase()) {
            // all this hideous nonsense calculates the various offsets for shadows
            case "drop":
                a.w = 0;
                a.l = a.t = o;
                a.t -= 1;
                if (Ext.isIE9m) {
                    a.l -= this.offset + rad;
                    a.t -= this.offset + rad;
                    a.w -= rad;
                    a.h -= rad;
                    a.t += 1;
                }
            break;
            case "sides":
                a.w = (o * 2);
                a.l = -o;
                a.t = o - 1;
                if (Ext.isIE9m) {
                    a.l -= (this.offset - rad);
                    a.t -= this.offset + rad;
                    a.l += 1;
                    a.w -= (this.offset - rad) * 2;
                    a.w -= rad + 1;
                    a.h -= 1;
                }
            break;
            case "frame":
                a.w = a.h = (o * 2);
                a.l = a.t = -o;
                a.t += 1;
                a.h -= 2;
                if (Ext.isIE9m) {
                    a.l -= (this.offset - rad);
                    a.t -= (this.offset - rad);
                    a.l += 1;
                    a.w -= (this.offset + rad + 1);
                    a.h -= (this.offset + rad);
                    a.h += 1;
                }
            break;
        };

        this.adjusts = a;
    };

    Ext.Shadow.prototype = {
        /**
         * @cfg {String} mode
         * The shadow display mode.  Supports the following options:<div class="mdetail-params"><ul>
         * <li><b><tt>sides</tt></b> : Shadow displays on both sides and bottom only</li>
         * <li><b><tt>frame</tt></b> : Shadow displays equally on all four sides</li>
         * <li><b><tt>drop</tt></b> : Traditional bottom-right drop shadow</li>
         * </ul></div>
         */
        /**
         * @cfg {String} offset
         * The number of pixels to offset the shadow from the element (defaults to <tt>4</tt>)
         */
        offset: 4,

        // private
        defaultMode: "drop",

        /**
         * Displays the shadow under the target element
         * @param {Mixed} targetEl The id or element under which the shadow should display
         */
        show: function(target) {
            target = Ext.get(target);
            if (!this.el) {
                this.el = Ext.Shadow.Pool.pull();
                if (this.el.dom.nextSibling != target.dom) {
                    this.el.insertBefore(target);
                }
            }
            this.el.setStyle("z-index", this.zIndex || parseInt(target.getStyle("z-index"), 10) - 1);
            if (Ext.isIE9m) {
                this.el.dom.style.filter = "progid:DXImageTransform.Microsoft.alpha(opacity=50) progid:DXImageTransform.Microsoft.Blur(pixelradius=" + (this.offset) + ")";
            }
            this.realign(
            target.getLeft(true),
            target.getTop(true),
            target.getWidth(),
            target.getHeight()
            );
            this.el.dom.style.display = "block";
        },

        /**
         * Returns true if the shadow is visible, else false
         */
        isVisible: function() {
            return this.el ? true: false;
        },

        /**
         * Direct alignment when values are already available. Show must be called at least once before
         * calling this method to ensure it is initialized.
         * @param {Number} left The target element left position
         * @param {Number} top The target element top position
         * @param {Number} width The target element width
         * @param {Number} height The target element height
         */
        realign: function(l, t, w, h) {
            if (!this.el) {
                return;
            }
            var a = this.adjusts,
                d = this.el.dom,
                s = d.style,
                iea = 0,
                sw = (w + a.w),
                sh = (h + a.h),
                sws = sw + "px",
                shs = sh + "px",
                cn,
                sww;
            s.left = (l + a.l) + "px";
            s.top = (t + a.t) + "px";
            if (s.width != sws || s.height != shs) {
                s.width = sws;
                s.height = shs;
                if (!Ext.isIE9m) {
                    cn = d.childNodes;
                    sww = Math.max(0, (sw - 12)) + "px";
                    cn[0].childNodes[1].style.width = sww;
                    cn[1].childNodes[1].style.width = sww;
                    cn[2].childNodes[1].style.width = sww;
                    cn[1].style.height = Math.max(0, (sh - 12)) + "px";
                }
            }
        },

        /**
         * Hides this shadow
         */
        hide: function() {
            if (this.el) {
                this.el.dom.style.display = "none";
                Ext.Shadow.Pool.push(this.el);
                delete this.el;
            }
        },

        /**
         * Adjust the z-index of this shadow
         * @param {Number} zindex The new z-index
         */
        setZIndex: function(z) {
            this.zIndex = z;
            if (this.el) {
                this.el.setStyle("z-index", z);
            }
        }
    };

    // Private utility class that manages the internal Shadow cache
    Ext.Shadow.Pool = function() {
        var p = [],
            markup = Ext.isIE9m ?
                '<div class="x-ie-shadow"></div>':
                '<div class="x-shadow"><div class="xst"><div class="xstl"></div><div class="xstc"></div><div class="xstr"></div></div><div class="xsc"><div class="xsml"></div><div class="xsmc"></div><div class="xsmr"></div></div><div class="xsb"><div class="xsbl"></div><div class="xsbc"></div><div class="xsbr"></div></div></div>';
        return {
            pull: function() {
                var sh = p.shift();
                if (!sh) {
                    sh = Ext.get(Ext.DomHelper.insertHtml("beforeBegin", document.body.firstChild, markup));
                    sh.autoBoxAdjust = false;
                }
                return sh;
            },

            push: function(sh) {
                p.push(sh);
            }
        };
    }();