Add Platform Sources using Netbeans Platform Manager
To be able to do Navigate ⇒ Go to Source on netbeans classes like TopComponent you need to link the sources to the Netbeans Plaform using the Netbeans Plaform Manager as explained in Hyperlinking to NetBeans API Sources. Be sure to download the proper source zip from the netbeans download page, otherwise the Platform Manager won’t let you attach the source file to the platform (all invalid options are greyed out/dimmed): ...