Could not figure out, how to do it in Inkscape.

Thanks a lot for the clarifications! So this was a Nimbus version problem.
As a side note, it is good to know that it does not solve the problem for users of the Nimbus external library.

I actually just fixed this bug and put it back this afternoon, it should show up in build 13 of Java 6 update 10, which I expect will be built and published in 3-4 weeks.

@Christopher:Oh my! There’s a build system running amok. We haven’t actually done any work on the code in nimbus.dev.java.net for months and months. All our work recently has been for Java 6 update 10, which meant we had to do it in the Teamware workspaces within Sun. We were bummed about it, and want to somehow get the work we’ve done backported to nimbus.dev.java.net, but not sure technically how to go about it (assume we get the legal permissions worked out!). I’ll be sure to disable the weekly builds on hudson so we don’t keep creating nonsense weekly builds!

The problem is that we’ve made upwards of 100 bugfixes in Synth and core Swing (including classes in the Basic LAF and some core Swing components and Borders). There is also one fix in java.awt.Color. Getting these somehow extracted into a Jar that could be used in nimbus.dev.java.net is going to be very hard. Even if we can do it, you would have to put the jar on the bootclasspath — something that would only really be legal to deploy under Java 7.

Once we get Java 6 update 10 out the door, the plan is to patch Java 7 to include all the fixes for Nimbus. We’re doing it this way so that we don’t have to maintain two code branches simultaneously. Once it is in Java 7, you can do whatever the GPL allows :-).

The root cause is located in NimbusComboBoxUI.paint(Graphics, JComponent):

if(renderer instanceof JComponent) {
((JComponent)renderer).setOpaque(false);
((JComponent)renderer).setForeground(comboBox.getForeground());
}

The wrapper renderer is not a JComponent and so the label that is returned ends up opaque.

This was tested with nimbus-2008_02_10.jar (latest weekly release) with Java 1.6.0_04.

-Christopher