You could try to add this static function

static void delayed_redraw(void *data) {
  ((Fl_Window *)data)->redraw();
}

and to override your GL3 window's resize method as follows:

  void resize(int x, int y, int w, int h)  override  {
    Fl_Gl_Window::resize(x, y, w, h);
    Fl::add_timeout(0.01, delayed_redraw, this);
  }