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. ...
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The Refactoring Browser (RB), which is renowned for its automated code refactoring capabilities. (Whereas Whisker is significant from purely a UI perspective.) The two browsers could be integrated together at some point, with Whisker as the UI, using the refactoring capabilities of the RB engine.
If you can't find a version of Whisker on SqueakMap or any other site, there are some versions here: http://dway.mailcan.com/whisker/
(These work primarily with Squeak versions 3.7 through 3.9. Whisker v1.2.0 is confirmed to work with Squeak v3.9.)