pretty much everything you need to develop a modern GUI application. All the code is open-source (GPL 3.0) and in the GitHub repo. While the original FabGL had a game-development orientation ...

the fact remains that for the majority of the world’s desktop computer users back then it would be the Microsoft Sound that heralded their first experience of a modern GUI operating system.

Either a stand-alone program or part of an application development system, GUI builders are a primary component in modern programming environments. See GUI and application development system.