The ListDemo.java in attachment creates a JList with one of its elements
having more than 20k characters. When the window pops up it is empty and
does not show any of the elements in the list but if you maximize the
window then you can see them.

I wasn't able to reproduce it on Tiger. It seems a regression in Mustang.

In order to reproduce it try the GTK (Solaris/SPARC) L&F, or the Motif L&F.
It is harder to reproduce on the Windows L&F.