I've noticed in prior releases, when cursor is over a Java built-in class name, like Graphics, MouseEvent, etc., the class definition source code displays in the Preview window. In B4, this behavior now displays the import statement for the class (quite useless) and I can navigate to the source code by pressing Cmd-. (Mac OS X key binding) upon which it gives me two selections to navigate to - the import in the current source file (not very interesting) and the actual source code in the src.zip file (which works when I select it).
