Swing L&Fs (for instance, Windows, GTK, and Nimbus) use rounded corners for regular components (like, say, buttons). When the components get mixed with heavyweight components, visual artifacts appear at the corners: they do not get rendered, though they get cut-out from the shapes of hw components.

To overcome the problem the L&Fs should specify the mixing-cutout-shape for the components using the new API provided with 6797587 in 6u14b02.