It isn't that common, horizontal scrollbars are generally frowned upon. It might be better to make the minimum width of your window such that this isn't necessary.
That said, a quick play suggests you might be able to achieve the desired effect with a bit of CSS and a little hacking of hbox:
innerCls: 'x-box-inner auto-scroll', // Undocumented config option
There may well be a neater way to do this with one of the other layouts and a bit of CSS.