CPU Usage at 100% when displaying an Expander

When displaying an Expander component, the CPU usage immediately goes up and stays at 100% as long as the component is displayed (even if the window is background, reduced or whatever).
To be precise, I really had 50% because I have a dual core : one of the core was 100%.

Reproduced on 3 different computers, all Windows XP, with Java 6 and 7.
Reproduced in applet and in standalone application shape.

Steps to reproduce :
1/ go to http://pivot.apache.org/tutorials/sample-application.html
-> CPU is normal, say 5%
2/ click on "navigation" to see the expander
-> CPU is at 100%, at least one of the cores.

Alternative : just go to http://pivot.apache.org/tutorials/expanders.html and expect the same.