The source code

    /**
     * @class Ext.layout.FitLayout
     * @extends Ext.layout.ContainerLayout
     * <p>This is a base class for layouts that contain <b>a single item</b> that automatically expands to fill the layout's
     * container.  This class is intended to be extended or created via the <tt>layout:'fit'</tt> {@link Ext.Container#layout}
     * config, and should generally not need to be created directly via the new keyword.</p>
     * <p>FitLayout does not have any direct config options (other than inherited ones).  To fit a panel to a container
     * using FitLayout, simply set layout:'fit' on the container and add a single panel to it.  If the container has
     * multiple panels, only the first one will be displayed.  Example usage:</p>
     * <pre><code>
    var p = new Ext.Panel({
        title: 'Fit Layout',
        layout:'fit',
        items: {
            title: 'Inner Panel',
            html: '&lt;p&gt;This is the inner panel content&lt;/p&gt;',
            border: false
        }
    });
    </code></pre>
     */
    Ext.layout.FitLayout = Ext.extend(Ext.layout.ContainerLayout, {
        // private
        monitorResize:true,

        type: 'fit',

        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.FitLayout.superclass.onLayout.call(this, ct, target);
            if(!ct.collapsed){
                this.setItemSize(this.activeItem || ct.items.itemAt(0), this.getLayoutTargetSize());
            }
        },

        // private
        setItemSize : function(item, size){
            if(item && size.height > 0){ // display none?
                item.setSize(size);
            }
        }
    });
    Ext.Container.LAYOUTS['fit'] = Ext.layout.FitLayout;