It was originally ported from a gtk example back in the early days. It may have been before we really had much subclassing support in Gtk#. I decided to see if gtk+ was still shipping the sample, and just do a clean port of the current sample instead of trying a minimal update.
The majority of the work needed to port the old sample to 3.0 boiled down to a Gdk.Pixmap to Cairo.Surface port. The ExposeEvent to Drawn event transition was needed as well. This is starting to sound a bit like a broken record, though I would imagine many readers have never heard a broken record.
But I digress.
The finished port of the gtk+ sample is now in master, along with all the fixes to the stack to expose some needed API that wasn't getting generated. The list of non-working samples is getting shorter.