diff -up gtk+-3.22.4/gtk/gtkcombobox.c build-i686-w64-mingw32/gtk/gtkcombobox.c
--- gtk+-3.22.4/gtk/gtkcombobox.c	2016-12-16 19:30:02.896074700 -0800
+++ build-i686-w64-mingw32/gtk/gtkcombobox.c	2017-01-03 09:35:27.267823400 -0800
@@ -1875,6 +1875,7 @@ gtk_combo_box_set_popup_widget (GtkCombo
             {
               gtk_window_group_add_window (gtk_window_get_group (GTK_WINDOW (toplevel)),
                                            GTK_WINDOW (priv->popup_window));
+              gtk_window_set_modal (GTK_WINDOW (priv->popup_window), TRUE);
               gtk_window_set_transient_for (GTK_WINDOW (priv->popup_window),
                                             GTK_WINDOW (toplevel));
             }
