Horizontal offset to apply to the menu, i.e. the rectangle or widget anchor.
See gtk_menu_popup_at_rect (), gtk_menu_popup_at_widget (), gtk_menu_popup_at_pointer (), #GtkMenu:anchor-hints, #GtkMenu:rect-anchor-dy, #GtkMenu:menu-type-hint, and #GtkMenu::popped-up.
Get rectAnchorDx property.