My understanding is the default coordinate space of cairo is:

    (0.0, 0.0) = top/left
    (1.0, 1.0) = bottom/right

..and to use FLTK's coordinate space, you'd use cairo_scale(cr, w(), h())
to convert to the window's integer coordinate space.

Here's what I have (it doesn't work - weird behavior).
Am I missing something?

#include <FL/Fl.H>
#include <FL/Fl_Cairo_Window.H>

static void my_cairo_draw_cb(Fl_Cairo_Window *window, cairo_t *cr) {
    const double xmax = (window->w() - 1);
    const double ymax = (window->h() - 1);

    // Draw green "X"
    //     Draw an X to four corners of resizable window.
    //
    cairo_scale(cr, window->w(), window->h());                    // use FLTK's coordinate space 0,0,w(),h()
    cairo_set_line_width(cr, 0.01);                               // line width for drawing
    cairo_set_source_rgb(cr, 0.0, 1.0, 0.0);                      // green
    cairo_move_to(cr, 0.0, 0.0);  cairo_line_to(cr, xmax, ymax);  // draw diagonal "\" in fltk coord space
    cairo_move_to(cr, 0.0, ymax); cairo_line_to(cr, xmax, 0.0);   // draw diagonal "/" in fltk coord space
    cairo_stroke(cr);                                             // stroke the lines
}

int main(int argc, char **argv) {
  Fl_Cairo_Window window(300, 300, "Cairo Draw 'X'");
  window.color(FL_BLACK);
  window.resizable(&window);
  window.set_draw_cb(my_cairo_draw_cb);
  window.show(argc, argv);
  return Fl::run();
}