GtkWidget draw event fired mutliple times

Question

I have been scratching my head on Why is below code triggers on-draw callback three times as opposed to just once.

#include <iostream>
#include <gtk/gtk.h>

using namespace std;

void on_draw(){
    cout << "drawing"<<endl;
}

int main( int argc, char *argv[])
{
  GtkWidget *window;
  GtkWidget *drgArea;

  gtk_init(&argc, &argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
  drgArea = gtk_drawing_area_new();
//  gtk_widget_set_redraw_on_allocate(drgArea, -1);
  gtk_container_add(GTK_CONTAINER(window), drgArea);
  gtk_widget_show_all(window);
  g_signal_connect(drgArea, "expose-event",
            G_CALLBACK(on_draw), NULL);
  gtk_main();

  return 0;
}

Answer 1

This is probably related to your compositor/window manager.

Using cinnamon [2.0.14] shows 2 redraws when starting up the application (no matter if I use gtk3/3.10.6/"draw" or gtk2/2.24.22/"expose-event").