With Visual Studio, you can visually inline the definition of a function below its call by selecting "Peek Definition", which is close to what you're saying.
You can only do it one at a time, and it's for quick scanning of what the function does, though.
Hmm, wonder if I can coax Sublime into doing that. It's a little closer to IDE-type functionality, which it's pretty weak on (refactoring is terrible for Ruby with it).
You can only do it one at a time, and it's for quick scanning of what the function does, though.