because if there's a tooltip visible then popups fail to show
with the following warning:
"Gdk-WARNING **: Tried to map a popup with a non-top most parent"

This bug affect popups of the form:
 1) popups from gtk_menu_popup_{at_widget|at_rect|for_device}()

    This can be reproduce with a normal GtkComboBox that has a
    tooltip attached (eg. via GtkBuilder 'tooltip_text' attribute).
    Also see GtkLabel reproducer from gtk3-demo mentioned in #1785

 2) custom popups created with gtk_window_new (GTK_WINDOW_POPUP)

    A reproducer that shows this case is a GtkComboBox with
    the property 'appears-as-list'[1] set to TRUE (default is FALSE).

Fixes issue #1785

[1] https://developer.gnome.org/gtk3/stable/GtkComboBox.html#GtkComboBox--s-appears-as-list