The source code

    /**
     * @class Ext.layout.AnchorLayout
     * @extends Ext.layout.ContainerLayout
     * <p>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
     * <b><tt>{@link #anchor}</tt></b> rules.</p>
     * <p>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.</p>
     * <p>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 <b>anchorSize</b>.
     * 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:</p>
     * <pre><code>
    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%'
        }]
    });
     * </code></pre>
     */
    Ext.layout.AnchorLayout = Ext.extend(Ext.layout.ContainerLayout, {
        /**
         * @cfg {String} anchor
         * <p>This configuation option is to be applied to <b>child <tt>items</tt></b> of a container managed by
         * this layout (ie. configured with <tt>layout:'anchor'</tt>).</p><br/>
         *
         * <p>This value is what tells the layout how an item should be anchored to the container. <tt>items</tt>
         * added to an AnchorLayout accept an anchoring-specific config property of <b>anchor</b> 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:<div class="mdetail-params"><ul>
         *
         * <li><b>Percentage</b> : Any value between 1 and 100, expressed as a percentage.<div class="sub-desc">
         * 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:<pre><code>
    // 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
         * </code></pre></div></li>
         *
         * <li><b>Offsets</b> : Any positive or negative integer value.<div class="sub-desc">
         * 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:<pre><code>
    // 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
         * </code></pre></div></li>
         *
         * <li><b>Sides</b> : Valid values are <tt>'right'</tt> (or <tt>'r'</tt>) and <tt>'bottom'</tt>
         * (or <tt>'b'</tt>).<div class="sub-desc">
         * 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.</div></li>
         *
         * <li><b>Mixed</b> : <div class="sub-desc">
         * 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:
         * <pre><code>
    anchor: '-50 75%'
         * </code></pre></div></li>
         *
         *
         * </ul></div>
         */

        // 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(), ret = {};
            if (target) {
                ret = target.getViewSize();

                // IE in strict mode will return a width of 0 on the 1st pass of getViewSize.
                // Use getStyleSize to verify the 0 width, the adjustment pass will then work properly
                // with getViewSize
                if (Ext.isIE9m && Ext.isStrict && ret.width == 0){
                    ret =  target.getStyleSize();
                }
                ret.width -= target.getPadding('lr');
                ret.height -= target.getPadding('tb');
            }
            return ret;
        },

        // private
        onLayout : function(container, target) {
            Ext.layout.AnchorLayout.superclass.onLayout.call(this, container, target);

            var size = this.getLayoutTargetSize(),
                containerWidth = size.width,
                containerHeight = size.height,
                overflow = target.getStyle('overflow'),
                components = this.getRenderedItems(container),
                len = components.length,
                boxes = [],
                box,
                anchorWidth,
                anchorHeight,
                component,
                anchorSpec,
                calcWidth,
                calcHeight,
                anchorsArray,
                totalHeight = 0,
                i,
                el;

            if(containerWidth < 20 && containerHeight < 20){
                return;
            }

            // find the container anchoring size
            if(container.anchorSize) {
                if(typeof container.anchorSize == 'number') {
                    anchorWidth = container.anchorSize;
                } else {
                    anchorWidth = container.anchorSize.width;
                    anchorHeight = container.anchorSize.height;
                }
            } else {
                anchorWidth = container.initialConfig.width;
                anchorHeight = container.initialConfig.height;
            }

            for(i = 0; i < len; i++) {
                component = components[i];
                el = component.getPositionEl();

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

                if(component.anchor) {
                    anchorSpec = component.anchorSpec;
                    // cache all anchor values
                    if(!anchorSpec){
                        anchorsArray = component.anchor.split(' ');
                        component.anchorSpec = anchorSpec = {
                            right: this.parseAnchor(anchorsArray[0], component.initialConfig.width, anchorWidth),
                            bottom: this.parseAnchor(anchorsArray[1], component.initialConfig.height, anchorHeight)
                        };
                    }
                    calcWidth = anchorSpec.right ? this.adjustWidthAnchor(anchorSpec.right(containerWidth) - el.getMargins('lr'), component) : undefined;
                    calcHeight = anchorSpec.bottom ? this.adjustHeightAnchor(anchorSpec.bottom(containerHeight) - el.getMargins('tb'), component) : undefined;

                    if(calcWidth || calcHeight) {
                        boxes.push({
                            component: component,
                            width: calcWidth || undefined,
                            height: calcHeight || undefined
                        });
                    }
                }
            }
            for (i = 0, len = boxes.length; i < len; i++) {
                box = boxes[i];
                box.component.setSize(box.width, box.height);
            }

            if (overflow && overflow != 'hidden' && !this.adjustmentPass) {
                var newTargetSize = this.getLayoutTargetSize();
                if (newTargetSize.width != size.width || newTargetSize.height != size.height){
                    this.adjustmentPass = true;
                    this.onLayout(container, target);
                }
            }

            delete this.adjustmentPass;
        },

        // 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;