Running the attached test case, the startup focus on JDK 1.6 is different
from the startup focus on JDK 1.5, for JInternalFrame.

Run the attached test case on JDK 1.6:
Click on the JInternalFrame.
The focus is on javax.swing.JPanel.

On JDK 1.5 the focus is on javax.swing.JButton,
so that is the expected result.

Note that a JInternalFrame created after startup with the
"New" menu item has the focus on javax.swing.JButton, same as JDK 1.5.