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The library description says that the src root is jar:nbinst://org.jdesktop.layout/docs/swing-layout-1.0.4-src.zip!/ but if you look into the zip file you will find that the source root is jar:nbinst://org.jdesktop.layout/docs/swing-layout-1.0.4-src.zip!/swing-layout-1.0.4/src/ It's really BAD that the bundled libraries have wrong description file.
You are right. It is a silly bug introduced with 1.0.4 version of the library. I replaced 1.0.3 files by 1.0.4 files downloaded from swing-layout.dev.java.net and I didn't realize that the file swing-layout-1.0.3-src.zip in the IDE has the same name as swing-layout-1.0.3-src.zip file on swing-layout.dev.java.net, but it is different. The file in the IDE was a stripped version of the original file. Hence, the source root changed when I introduced the original (non-stripped) file. This issue IMHO is not a blocker for NB 6.8 release (e.g. will not be fixed as P1 before FCS), but it is a very good candidate for the first patch.
Fixed. Modified file: http://hg.netbeans.org/cdev/rev/34c570bd2e57