I filed this yesterday https://github.com/apache/netbeans/issues/5654 and added an example today.
As this might be an odd corner case I tried to do a workaround adding the InputOutput to my own TopComponent as described in https://netbeans.apache.org/wiki/DevFaqOWTabEmbedding.html I get a working output but only with ONE tab, and the page above only mentions a singular tab. How do I get multiple tabs in my custom TopComponent output? /Patrik