WigglyPaint: A juicy, jiggly drawing program
internet-janitor.itch.io
internet-janitor.itch.io
Various other buttons and interactive elements contain their own scripts; from the main view in "Widgets" editing mode you can double-click on the ball-point pen tool and then click "Script..." to see how tool-changing works, for example.
That said, anyone interested in building tooling is welcome to do so. This is why, for example, the Decker file format is fully documented: https://beyondloom.com/decker/format.html