Last updated at 1:45 am UTC on 5 January 2017
Plumbin' was created by Ward Cunningham back around 1985 in Tektronix Smalltalk as an exercise to show how much can be done with MVC, and how easily.
A modern Morphic version now exists, primarily as an example in using drag and drop within Morphic.
You can load this from squeaksource
Jump over to Drag and Drop Explained to learn about how drag and drop works.
Ward did update his original for Squeak in '97 (and added colour); this version started from his code and modified quite a lot.