The source code

    /**
     * @class Ext.layout.FitLayout
     * @extends Ext.layout.ContainerLayout
     * This is a base class for layouts that contain a single item that automatically expands to fill the layout's
     * container.  This class is intended to be extended or created via the layout:'fit' {@link Ext.Container#layout}
     * config, and should generally not need to be created directly via the new keyword.
     * 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:
     *
    var p = new Ext.Panel({
        title: 'Fit Layout',
        layout:'fit',
        items: {
            title: 'Inner Panel',
            html: '<p>This is the inner panel content</p>',
            border: false
        }
    });

     */
    Ext.layout.FitLayout = Ext.extend(Ext.layout.ContainerLayout, {
        // private
        monitorResize:true,

        // private
        onLayout : function(ct, target){
            Ext.layout.FitLayout.superclass.onLayout.call(this, ct, target);
            if(!this.container.collapsed){
                var sz = (Ext.isIE6 && Ext.isStrict && target.dom == document.body) ? target.getViewSize() : target.getStyleSize();
                this.setItemSize(this.activeItem || ct.items.itemAt(0), sz);
            }
        },

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