Nimbus L&F:Grey Pixel appearing at the edges of Inacive JInternalFrames

This is observed on Nimbus L&F on all OSes, with the current 6u4b02 pit build.
Inactive JInternal Frame's Menu drop down (restore,Maximize,Minimize etc.) appears with a single grey colored pixel at the 4 curved edges of the InternalFrame.
This is visible the first time when you bring up the application. If you click on the InternalFrame to make it active and then make it inactive again -it appears fine.
Screen shot is attached for reference.