The attached test fullscreens a window before showing it. It will receive 3 state changes instead of one, and this is a problem when trying to setup a fullscreen window from the application start, as it will cause flickering in the display.

$ gcc -Wall -g -o test test.c `pkg-config --libs --cflags gtk+-2.0`
$ ./test # Wait 5 seconds
** Message: it says we are fullscreen now
** Message: it says we're not fullscreen now
** Message: it says we are fullscreen now

test.c inline:
---8<---
#include <gtk/gtk.h>
#include <stdlib.h>

static gboolean
window_state_event_cb (GtkWidget *window, GdkEventWindowState *event,
                       gpointer user_data)
{
        if ((event->changed_mask & GDK_WINDOW_STATE_FULLSCREEN) == 0)
                return FALSE;

        if (event->new_window_state & GDK_WINDOW_STATE_FULLSCREEN) {
                g_message ("it says we are fullscreen now");
        } else {
                g_message ("it says we're not fullscreen now");
        }

        return FALSE;
}

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

        gtk_init (&argc, &argv);

        window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
        g_signal_connect (G_OBJECT (window), "window-state-event",
                          G_CALLBACK (window_state_event_cb), NULL);

        gtk_window_fullscreen (GTK_WINDOW(window));

        gtk_widget_show (window);

        g_timeout_add (5000, exit, NULL);

        gtk_main ();

        return 0;
}
---8<---