Please describe the problem:
While trying to change a window from one screen to another, if the window contains a GtkComboboxEntry that "appears-as-list". The GtkComboboxEntry's dropdown list always appears on  the first screen of the window (whixh is not the screen of the window)

Steps to reproduce:
1. Create a Window with a combobox that "appear as list" (see sample code)
2. Open the dropdown list
3. change the screen of the window
4. Open the dropdown list


Actual results:
On step 4, the dropdown appears on the first screen

Expected results:
The dropdown window should appear on the combobox's window.

Does this happen every time?
yes 

Other information:
here is a sample code that can allow you to reproduce the bug : 
////
#include <gtk/gtk.h>

static GdkScreen* _swap_screen(GdkScreen* anInputScreen)
{
  GdkDisplay * display = gdk_display_get_default (); 
  int num_screen = gdk_display_get_n_screens (display);
  int input_screen_num = gdk_screen_get_number(anInputScreen);
  return gdk_display_get_screen(display, ++input_screen_num % num_screen);
}

static void _change_screen_callback(GtkWidget* aButton, gpointer aUserData)
{
  GtkWidget *aWindow = GTK_WIDGET(aUserData);
  GdkScreen * aScreen = gtk_window_get_screen(GTK_WINDOW(aWindow));
  aScreen = _swap_screen(aScreen);
  gtk_window_set_screen(GTK_WINDOW(aWindow), aScreen);
}

static gboolean quit_callback(GtkWidget *widget,
			      GdkEvent *event,
			      gpointer user_data)
{
  gtk_main_quit();
  return FALSE;
}

void create_widget(GtkWidget* aMainWindow)
{
  // container
  GtkWidget* aVbox = gtk_vbox_new(FALSE, 3);
  gtk_container_add(GTK_CONTAINER(aMainWindow), aVbox);
  // combo
  
  // set appears as list property
  GtkWidget* aWidget = gtk_combo_box_entry_new();
  gtk_widget_class_install_style_property 
    ((GtkWidgetClass *)(GTK_COMBO_BOX_ENTRY_GET_CLASS(aWidget)), 
     g_param_spec_boolean("appears-as-list",
			  NULL, NULL, TRUE, G_PARAM_READABLE));
  gtk_object_sink(GTK_OBJECT(aWidget));
/// done

  GtkWidget* aCombobox = gtk_combo_box_entry_new_text();
  gtk_combo_box_prepend_text(GTK_COMBO_BOX(aCombobox),
			     "Item 1");
  gtk_combo_box_prepend_text(GTK_COMBO_BOX(aCombobox),
			     "Item 2");
  gtk_combo_box_prepend_text(GTK_COMBO_BOX(aCombobox),
			     "Item 3");
  gtk_box_pack_start(GTK_BOX(aVbox),
		     aCombobox, TRUE, TRUE, 0);
 

  // separator
  GtkWidget* aSeparator = gtk_hseparator_new();
  gtk_box_pack_start(GTK_BOX(aVbox),
		     aSeparator, FALSE, FALSE, 0);
  // button
  GtkWidget* aChangeScreenButton = gtk_button_new_with_label("change screen");
  g_signal_connect ( G_OBJECT(aChangeScreenButton), "clicked", G_CALLBACK(_change_screen_callback), aMainWindow);
  
  gtk_box_pack_start(GTK_BOX(aVbox),
		     aChangeScreenButton, TRUE, TRUE, 0);
 
}

int main(int argc, char** argv)
{
    gtk_disable_setlocale();
    gtk_init(&argc,&argv);
    GtkWidget *aMainWindow = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    g_signal_connect(G_OBJECT(aMainWindow), "delete-event", G_CALLBACK(quit_callback), NULL);
    create_widget(aMainWindow);
    gtk_widget_show_all(aMainWindow);
    gtk_main();
    return 0;
}

///End Code

I think this problem can be solved by adding the command (not tested...)
 gtk_window_set_screen (GTK_WINDOW (combo_box->priv->popup_window),
                                 gtk_widget_get_screen (GTK_WIDGET (combo_box)));

in the function gtk_combo_box_popup (line 1618 of gtkcombobox.c in Gtk 2.6.10)