However there can be two problems with it, the first is that the busy animation is just dreadful. When you set the progress bar to that mode it animates a rectangle bouncing from side to side. There's something very reminiscent of Windows 95 in that, and it's not a very nice thing to look at.
The second problem is that it always seem to obey the minimum size specified on the theme, and so a programmer can't choose how small to make the progressbar.
So the only answer was to make a custom gtk control for progress and busy animations.
Here it is as a progress bar and a busy bar in various sizes and types.
In Data Forms Action the progress bar and busy bar are used when opening a page and refreshing the data sources on it for the first time. Therefore the progress bar has to show the progress as connections or queries finish, but also display a busy bar whilst the connections or queries are in progress, so the progress bar I developed here allows you to switch between the two.
For the visual effect I've borrowed quite shamelessly from Mac OS X and used a barberpole animation as well as rounded corners.