#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Button.H>
#include <FL/Fl_Widget.H>

void btn_cb(Fl_Widget *, void *v) {
    Fl_Window *w = (Fl_Window *)v;
    if (!w->fullscreen_active()) w->fullscreen();
    else                         w->fullscreen_off();
}

int main(int argc, char **argv) {
  Fl_Window *window = new Fl_Window(1920, 1200);
  Fl_Button *btn = new Fl_Button(0, 0, 128, 32, "fullscreen");
  btn->callback(btn_cb, window);
  window->end();
  window->resizable(window);
  window->fullscreen();
  window->show(argc, argv);
  return Fl::run();
}