org.openpatch.scratch.extension
Everything in here works in a project you run on your own computer, but not in the examples you can start in the browser. There is no file system, no video encoder and no OpenGL behind a web page, so calling it there tells you as much and does nothing.
The public extension API: what an extension jar uses to add to Scratch for Java without touching its internals. First-party extensions (tiled, save, recorder, drawing) are built on it alone, which the build checks.
Desktop only: the online IDE has every first-party extension built in and loads no others.