How about a few more [b]Spring[/b]s
I've been using SpringLayout recently and it seem to me it could be a lot more versatile with a few extra Spring functions.
I've created Spring classes which track the actual rather than the prefferred height or width of an object. This allows me to position components as a proportion of the container size, and so that they track the container size.
Sometimes we could also do with minimum and maximum springs, for example to determine a preffered hieght of a container when we don't know up front which of two components wants to be taller.