Look up "class browser", in particular "smalltalk class browser" to see examples, current and historic, of editors supporting that kind of approach to navigating codebases.
Maybe I'm looking up the wrong thing but these basically list the classes and method names and hotlink to the code?
I mean something that would dump them all into, one contiguous "file" from the editor's perspective. Included components (I don't want to say classes because it could work in a functional language or maybe you just don't include full classes but certain methods) wouldn't have to be coupled to the main class you're editing. Like if you have a decoupled event system you could pull in just the events relevant to the idea you're working on. You could have different views depending on what idea you're working on and save them as their own file.
To use the gamedev analogy again you could have MonsterCombat.view MonsterAI.view MonsterAnimations.view which would all expose different subsets of the Monster class and various related methods from other classes/modules.
