Nimbus L&F:Active Inactive Internal JFrame look too similar.
To view this issue look at the Internal frames in the JInternal frame demo in SwingSet2.
Observe that there is very little difference between an inactive and active JInternalFrame.
Since this is very subjective, I have attached a screen shot of active JInternalFrame overlapping over an inactive JInternalFrame in Winxp l&F, Motif, Metal , Nimbus L&F and GTK BluePrint L&F as well as Two native command terminals in Nimbus Theme.
Nimbus Look and Feel the differnce is least obvious of all (despite the color difference in the Maximize,Minimize,etc. buttons and the drop dwon menu at the top left).

The difference is very obvious in WinxP and Motif L&F,GTK due to strong difference in colors fo the Window decoration Same is true for Metal although to a less extent.
Native Nimbus theme is very similar to NimbusL&F in Java but since the title of the inactive Command terminal is grayed out, the difference is more obvious in native.
Either we should increase the color difference between the two(this would match native less) or the title text should also be greyed out(closer to native nimbus but looks a bit odd) or any other approach - so that the difference is more prominent.