There's a lot of architectural influence on the new CodeMirror from our ProseMirror work, but the actual systems are different enough that code sharing didn't really work (or, at least, would add enough levels of extra indirection and complexity to the reused code that it's not worth it—code size is a significant cost in JS components)

This makes me wonder. What would happen if you'd embed a piece of code inside a text document? Would you need to embed a CodeMirror instance inside a ProseMirror instance?

Have a look at https://prosemirror.net/examples/codemirror/ :)