Thanks to a cool tip from Steven Yi (here in the comments to a blog entry), I was able to make a totally pointless but fun set of draggable and resizable editors:
What you see above are two JEditorPanes within JPanels. The JPanels are within ComponentWidgets provided by the NetBeans Visual Library, which is also where the special border comes from. The ComponentWidgets are within a Visual Library Scene, which is within a JScrollPane in a TopComponent.
Each editor has this, which means the NetBeans Java Editor is bound to the JEditorPane:
EditorKit kit = CloneableEditorSupport.getEditorKit("text/x-java");
A similar thing is done in the other JEditorPane, i.e., it is bound to the XML Editor. While the XML Editor also has code completion, in addition to syntax coloring, as can be seen above, this is not the case for the JEditorPane bound to the Java Editor, since the JEditorPane doesn't have a Java classpath, which is needed for Java code completion to work.