#include <FL/Fl.H>

#include <FL/Enumerations.H>
#include <FL/Fl_Box.H>
#include <FL/Fl_Double_Window.H>
#include <FL/fl_draw.H>

class Canvas : public Fl_Box {
  Fl_Offscreen offs = 0;
  int ox = 0, oy = 0;

public:
  Canvas(int x, int y, int w, int h)
      : Fl_Box(x, y, w, h), offs(fl_create_offscreen(w, h)) {
    fl_begin_offscreen(offs);
    fl_color(FL_WHITE);
    fl_rectf(0, 0, w, h);
    fl_end_offscreen();
  }
  void draw() override {
    if (offs) {
      fl_rescale_offscreen(offs);
      fl_copy_offscreen(x(), y(), w(), h(), offs, 0, 0);
    }
    Fl_Box::draw();
  }
  int handle(int e) override {
    auto ret = Fl_Box::handle(e);
    switch (e) {
    case FL_PUSH: {
      fl_begin_offscreen(offs);
      fl_color(FL_RED);
      fl_line_style(FL_SOLID, 3);
      ox = Fl::event_x();
      oy = Fl::event_y();
      fl_point(ox, oy);
      fl_end_offscreen();
      redraw();
      fl_line_style(FL_SOLID, 0);
      return true;
    }
    case FL_DRAG: {
      fl_begin_offscreen(offs);
      fl_color(FL_RED);
      fl_line_style(FL_SOLID, 3);
      int nx = Fl::event_x();
      int ny = Fl::event_y();
      fl_line(ox, oy, nx, ny);
      ox = nx;
      oy = ny;
      fl_end_offscreen();
      redraw();
      fl_line_style(FL_SOLID, 0);
      return true;
    }
    default:
      return ret;
    }
  }
};

int main() {
  auto win = new Fl_Window(800, 600);
  auto box = new Canvas(5, 5, 790, 590);
  win->end();
  win->show();
  return Fl::run();
}