The HW/LW Mixing feature has been integrated into 6u12. Along with the fixes that were already present in JDK 7, there was a number of enhancements. These need to be forward-ported to JDK7. The list of additional fixes:

Fixed
6u12
6637655 Mixing of heavyweight/lightweight components does not wrk with GlassPane? childre
6768332 PIT : Internal frame can be seen below the Canvas
6769511 PIT : AWT components are invisible for a while after frame is moved & menu items are visible

6768230 HW/LW mixing code slows down resize performance
6777320 PIT : Canvas is not fully painted on the internal frame & internal frame goes behind the canvas
6777370 PIT : AWT components are not rendered correctly after resizing the JFrame.

6u14
6776743 Lightweight components must be counted as opaque rectangles for the purposes of hw/lw mixing
6779670 Recursive procedures in the HW/LW Mixing code must traverse parent containers
6788954 Need to introduce sun.awt.disableMixing system property
6797587 Need better control over shapes cut out by hw/lw mixing
6789096 HW/LW Mixing: 2% regression in 6u12 b01, alacrity SwingMark benchmark