IIRC, that was one of the features of the Whisker Browser back in the day.

http://wiki.squeak.org/squeak/1993


The goal of the Whisker Browser (a.k.a. Stacking Browser) is to provide 
a simple and intuitive way to view the contents of multiple classes and 
multiple methods simultaneously, while using screen real estate 
efficiently and not requiring a lot of window moving/resizing. It does 
this by introducing the concept of subpane stacking. ...


Cheers,
Bob

On 1/2/13 5:32 PM, Frank Shearar wrote:
> The only advantage I still see of lots-of-stuff-inna-file is that you
> can very quickly hop around a bunch of code. Our tools just don't work
> that way. They_could_. Noone's just ever hurt enough to display code
> in this fashion. It's easy enough: what's not so easy is to make that
> big blob of text efficiently editable such that you still keep track
> of the, for example, individual methods.

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