The attached screenshot shows a JInternalFrame on the "default" layer with a
JInternalFrame on the "modal" layer on top. The window in front is being
obscured by components on the window below. (The window in front is not
modal, it is merely on the modal layer.) This is the Windows L&F.

The JInternalFrame in the back has a JPanel as one of its descendants. This
JPanel has GridBagLayout(2,3). There are JRadioButtons in cells (2, 1) and
(2, 2). All the other cells contain (new JLabel("")).

The four empty labels are the ones that are being drawn such that they
obscure the window in the modal layer.

The problem occurs regardless of which window is active. It continues to
occur as I drag either window around. As I drag the background window,
artifacts from the grey rectangles are left behind on the desktop pane.
When I stop dragging, they get cleaned up.