Looks neat. But I was bummed out by this: "Frames are useful for statements, but they get unwieldy if you were to use them for expressions." I'd love to see an approach to this that works for languages that don't have a statement / expression distinction.
Yeah, it sounds like what they really mean is "we want our frames to be course-grained, because they are still a bit awkward to work with".
You can reflect the entire AST in your structure editor, so all text is parsed (just identifiers, or perhaps keywords). But then since basically all editor actions involve manipulating frames/ast nodes, you need to make the UX extremely smooth -- shortcut keys for high-level actions alone won't cut it.
