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