It has always been my belief that the GUI API of all modern systems is much too high level. Toolkits (even FL) are not what should be provided and documeted as part of an operating system. The system only has to provide arbitrary shaped but featureless windows, a powerful set of graphics drawing calls, and a simple unalterable method of delivering events to the owners of the windows. NeXT (if you ignored NextStep) provided this, but they chose to hide it and tried to push their own baroque toolkit instead...
Many of the ideas in fltk were developed on a NeXT (but not using NextStep) in 1987 in a C toolkit I called "views". Here I came up with passing events downward in the tree and having the handle routine return a value indicating the used the event, and the table-driven menus. In general I was trying to prove that complex UI ideas could be entirely implemented in a user space toolkit, with no knowledge or support by the system.
After going to film school for a few years, I worked at Sun Microsystems on the (doomed) NeWS project. Here I found an even better and cleaner windowing system, and I reimplemented "views" atop that. NeWS did have an unnecessarily complex method of delivering events which hurt it. But the designers did admit that perhaps the user could write just as good of a button as they could, and officially exposed the lower level interface.
With the death of NeWS I realized I would have to live with X. The biggest problem with X is the "window manager", which means that the toolkit can no longer control the window borders or drag the window around.
At Digital Domain I discovered another toolkit, "Forms". Forms was similar to my work, but provided many more widgets, since it was used in many real applications, rather then as theoretical work. I decided to use Forms, except I integrated my table-driven menus into it. Several very large programs were created using this version of Forms.
The need to switch to OpenGL and GLX, portability, and a desire to use C++ subclassing required a rewrite of Forms. This produced the first version of fltk. The conversion to C++ required so many changes it made it impossible to recompile any Forms objects. Since it was incompatable anyway, I decided to incorporate as much as possible my older ideas on simplifying the lower level interface and the event passing mechanisim.
I received permission to release it for free on the Internet, with the GNU general public license. Response from Internet users indicated that the Linux market dwarfed the SGI and high-speed GL market, so I rewrote it to use X for all drawing, greatly speeding it up on these machines. That is the version you have now.