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.layout.AnchorLayout
     * @extends Ext.layout.ContainerLayout
     * This is a layout that enables anchoring of contained elements relative to the container's dimensions.
     * If the container is resized, all anchored items are automatically rerendered according to their
     * {@link #anchor} rules.
     * This class is intended to be extended or created via the layout:'anchor' {@link Ext.Container#layout}
     * config, and should generally not need to be created directly via the new keyword.
     * AnchorLayout does not have any direct config options (other than inherited ones). By default,
     * AnchorLayout will calculate anchor measurements based on the size of the container itself. However, the
     * container using the AnchorLayout can supply an anchoring-specific config property of anchorSize.
     * If anchorSize is specifed, the layout will use it as a virtual container for the purposes of calculating
     * anchor measurements based on it instead, allowing the container to be sized independently of the anchoring
     * logic if necessary.  For example:
     *
    var viewport = new Ext.Viewport({
        layout:'anchor',
        anchorSize: {width:800, height:600},
        items:[{
            title:'Item 1',
            html:'Content 1',
            width:800,
            anchor:'right 20%'
        },{
            title:'Item 2',
            html:'Content 2',
            width:300,
            anchor:'50% 30%'
        },{
            title:'Item 3',
            html:'Content 3',
            width:600,
            anchor:'-100 50%'
        }]
    });
     *
     */
    Ext.layout.AnchorLayout = Ext.extend(Ext.layout.ContainerLayout, {
        /**
         * @cfg {String} anchor
         * This configuation option is to be applied to child items of a container managed by
         * this layout (ie. configured with layout:'anchor').

         *
         * This value is what tells the layout how an item should be anchored to the container. items
         * added to an AnchorLayout accept an anchoring-specific config property of anchor which is a string
         * containing two values: the horizontal anchor value and the vertical anchor value (for example, '100% 50%').
         * The following types of anchor values are supported:
         *
         * Percentage : Any value between 1 and 100, expressed as a percentage.
         * The first anchor is the percentage width that the item should take up within the container, and the
         * second is the percentage height.  For example:
    // two values specified
    anchor: '100% 50%' // render item complete width of the container and
                       // 1/2 height of the container
    // one value specified
    anchor: '100%'     // the width value; the height will default to auto
         *
         *
         * Offsets : Any positive or negative integer value.
         * This is a raw adjustment where the first anchor is the offset from the right edge of the container,
         * and the second is the offset from the bottom edge. For example:
    // two values specified
    anchor: '-50 -100' // render item the complete width of the container
                       // minus 50 pixels and
                       // the complete height minus 100 pixels.
    // one value specified
    anchor: '-50'      // anchor value is assumed to be the right offset value
                       // bottom offset will default to 0
         *
         *
         * Sides : Valid values are 'right' (or 'r') and 'bottom'
         * (or 'b').
         * Either the container must have a fixed size or an anchorSize config value defined at render time in
         * order for these to have any effect.
         *
         * Mixed :
         * Anchor values can also be mixed as needed.  For example, to render the width offset from the container
         * right edge by 50 pixels and 75% of the container's height use:
         *
    anchor: '-50 75%'
         *
         *
         *
         *
         */

        // private
        monitorResize : true,

        type : 'anchor',

        /**
         * @cfg {String} defaultAnchor
         *
         * default anchor for all child container items applied if no anchor or specific width is set on the child item.  Defaults to '100%'.
         *
         */
        defaultAnchor : '100%',

        parseAnchorRE : /^(r|right|b|bottom)$/i,

        getLayoutTargetSize : function() {
            var target = this.container.getLayoutTarget();
            if (!target) {
                return {};
            }
            // Style Sized (scrollbars not included)
            return target.getStyleSize();
        },

        // private
        onLayout : function(ct, target){
            Ext.layout.AnchorLayout.superclass.onLayout.call(this, ct, target);
            var size = this.getLayoutTargetSize();

            var w = size.width, h = size.height;

            if(w < 20 && h < 20){
                return;
            }

            // find the container anchoring size
            var aw, ah;
            if(ct.anchorSize){
                if(typeof ct.anchorSize == 'number'){
                    aw = ct.anchorSize;
                }else{
                    aw = ct.anchorSize.width;
                    ah = ct.anchorSize.height;
                }
            }else{
                aw = ct.initialConfig.width;
                ah = ct.initialConfig.height;
            }

            var cs = this.getRenderedItems(ct), len = cs.length, i, c, a, cw, ch, el, vs, boxes = [];
            for(i = 0; i < len; i++){
                c = cs[i];
                el = c.getPositionEl();

                // If a child container item has no anchor and no specific width, set the child to the default anchor size
                if (!c.anchor && c.items && !Ext.isNumber(c.width) && !(Ext.isIE6 && Ext.isStrict)){
                    c.anchor = this.defaultAnchor;
                }

                if(c.anchor){
                    a = c.anchorSpec;
                    if(!a){ // cache all anchor values
                        vs = c.anchor.split(' ');
                        c.anchorSpec = a = {
                            right: this.parseAnchor(vs[0], c.initialConfig.width, aw),
                            bottom: this.parseAnchor(vs[1], c.initialConfig.height, ah)
                        };
                    }
                    cw = a.right ? this.adjustWidthAnchor(a.right(w) - el.getMargins('lr'), c) : undefined;
                    ch = a.bottom ? this.adjustHeightAnchor(a.bottom(h) - el.getMargins('tb'), c) : undefined;

                    if(cw || ch){
                        boxes.push({
                            comp: c,
                            width: cw || undefined,
                            height: ch || undefined
                        });
                    }
                }
            }
            for (i = 0, len = boxes.length; i < len; i++) {
                c = boxes[i];
                c.comp.setSize(c.width, c.height);
            }
        },

        // private
        parseAnchor : function(a, start, cstart){
            if(a && a != 'none'){
                var last;
                // standard anchor
                if(this.parseAnchorRE.test(a)){
                    var diff = cstart - start;
                    return function(v){
                        if(v !== last){
                            last = v;
                            return v - diff;
                        }
                    }
                // percentage
                }else if(a.indexOf('%') != -1){
                    var ratio = parseFloat(a.replace('%', ''))*.01;
                    return function(v){
                        if(v !== last){
                            last = v;
                            return Math.floor(v*ratio);
                        }
                    }
                // simple offset adjustment
                }else{
                    a = parseInt(a, 10);
                    if(!isNaN(a)){
                        return function(v){
                            if(v !== last){
                                last = v;
                                return v + a;
                            }
                        }
                    }
                }
            }
            return false;
        },

        // private
        adjustWidthAnchor : function(value, comp){
            return value;
        },

        // private
        adjustHeightAnchor : function(value, comp){
            return value;
        }

        /**
         * @property activeItem
         * @hide
         */
    });
    Ext.Container.LAYOUTS['anchor'] = Ext.layout.AnchorLayout;