The delay is called "hysteresis". Gtk implements this through "gtk-menu-popdown-delay", and seems to be making sure the mouse is moving in a triangle region tow

The delay is called "hysteresis". Gtk implements this through "gtk-menu-popdown-delay", and seems to be making sure the mouse is moving in a triangle region towards the submenu. (See gtk/gtkmenu.c, esp gtk_menu_set_submenu_navigation_region.)