I would like to be able to create a preview window that shows non-editable preview of the subgraph contents. This preview might show when hovering over the collapsed item or, if that is too difficult, when double-clicking the collapsed item.
You can use override doUncpaturedMouseMove either on your subgraphs or on the JGoView to create the overview window when the mouse is moved over the subgraphs.
You would want to use a JGoView to implement your preview window. The JGoView would need to be associated with a different JGoDocument because the JGoView simply shows whatever is in the JGoDocument and your subgraph can't be simultaneously expanded and not expanded. So you would need to create a completely separate JGoView (with its associated JGoDocument) and make a copy of the group in the new document. Then expand the group and hopefully all will be well.
We ended up producing an image containing a snapshot of the uncollapsed group and use that as the source of our preview window. This did involve quite a bit of custom code, but was not too bad.