try this

Code:
Ext.ux.Box = Ext.extend(Ext.Container, {
    initComponent: function() {
        this.autoEl = {};
        Ext.ux.Box.superclass.initComponent.apply(this, arguments);
    },

    onRender: function(c) {
        Ext.ux.Box.superclass.onRender.apply(this, arguments);
        this.body = this.layoutTarget = this.el;
        this.el = this.body.boxWrap();
        this.bwrap = Ext.get(this.body.dom.parentNode);
        this.top = this.el.child('.x-box-tl');
        this.bottom = this.el.child('.x-box-bl');
        this.ml = this.el.child('.x-box-ml');
        this.mr = this.ml.child('.x-box-mr');
        this.mc = this.mr.child('.x-box-mc');
        if (this.title) {
            this.header = this.bwrap.createChild({tag:'h3', html: this.title }, this.body);
        }
        if (this.resizable) {
            this.resizer = new Ext.Resizable(this.el, {handles:'s,se,e'});
        }
    },

    getLayoutTarget: function() {
        return this.body;
    },
    
    getFrameHeight: function() {
        var result = 0;
        if (this.rendered) {
            result = this.el.getPadding('tb') + this.top.getHeight() + this.bottom.getHeight() + this.mc.getPadding('tb');
            if (this.header) {
                result += this.header.getHeight();
            }
        }
        return result;
    },

    getFrameWidth: function() {
        var result = 0;
        if (this.rendered) {
            result = this.el.getPadding('lr') + this.ml.getPadding('lr') + this.mc.getPadding('lr') + this.mr.getPadding('lr');
        }
        return result;
    },

    onResize : function(w, h){
        if(w !== undefined || h !== undefined){
            if(typeof w == 'number'){
                this.body.setWidth(w - this.getFrameWidth());
            }else if(w == 'auto'){
                this.body.setWidth(w);
            }

            if(typeof h == 'number'){
                this.body.setHeight(h - this.getFrameHeight());
            }else if(h == 'auto'){
                this.body.setHeight(h);
            }
        }
    }
});
Ext.reg('ux-box', Ext.ux.Box);