I have a JFrame which contains desktopPane. The DesktopPane contains two internal frame, which intern contains AWT Canvas. When i move the one internal frame to another , the internal frame goes behind the other internalframe canvas. This is not seen in metal look and feel, but it is seen very clear if i run the testcase either in windows , nimbus & GTK LookAndFeel. I have attached the screen shot & testcase.

Step to reproduce:-
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1) Run the attached testcase in any look and feel ( windows, nimbus & GTKLookAndFeel) -Dswing.defaultlaf=com.sun.java.swing.plaf.windows.WindowsLookAndFeel for windows laf
 -Dswing.defaultlaf=com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel for nimbus
2) When the gui is visible, click on the red internal frame, observe that portion of the internal frame is hidden with in another canvas. If you see the same then the bug is reproduced.
3) Move one internal frame with in another internal frame, observe that the top internal frame is gets hidden the below internal frame's canvas.

I tested this in 6u12 b01 pit build.