In Risc OS source release, thenot executable.
Basic programs have been converted to C using OSlib.
https://www.riscosopen.org/content/downloads/risc-os-tarballs 

Desktop Wimp Test: Program to test multiple nesting window opening

A series of windows is moved in a circle. If the set of rectangles to be copied forms a circle, one of the acorns should flicker, as it won't be copiable and must be redrawn. This is an improvement over the old Wimp, which would have to redraw ALL the acorns.

Multi : download 
Multi source : download