It seems that source attachement for Library Wrapper Modules in Netbeans 6.8 Beta doesn’t work properly. I opened a bug report here https://netbeans.org/bugzilla/show_bug.cgi?id=176800#c9 and it seems that in the process I also spotted that “Swing Layout Extensions” library definition that comes with NB 6.8 it’s also wrong (see https://netbeans.org/bugzilla/show_bug.cgi?id=176800#c7)