In fact, too helpful. Or, rather, it was so precisely what I needed, that I wonder why it was necessary for me to copy so much source code verbatim, rather than just invoking some higher level API.
If you have a file type that can be edited in 2 ways (text and some other GUI), and you've already written TopComponents for both, you can pretty much just paste in a big chunk of the minicomposer ScoreEditorSupport to your own EditorSupport, and pull in a little code from ScorePanel to your own GUI TopComponent. You may want to change the names of the buttons ("compose", for me, became "graph"). But that's about it.
Extremely simple, but not exactly the best example of code reuse.