Make progress bar more robust to window size change

At the moment, when using the ``ProgressBar`` class, if a window is resized, things go very wrong:

![Screen Shot 2013-03-05 at 17 12 07 ](https://f.cloud.github.com/assets/314716/221988/98f05b48-85af-11e2-9368-50107c815596.png)

Maybe there's not much that can be done about this, but just wanted to raise this in case there's an easy fix?