gtk-popover-menu-open-submenu (popover name)

popover -- a gtk-popover-menu widget
name -- a string with the name of the menu to switch to

Opens a submenu of the popover. The name must be one of the names given to the submenus of the popover with "submenu", or "main" to switch back to the main menu.

The gtk-model-button widget will open submenus automatically when the menu-name property is set, so this function is only needed when you are using other kinds of widgets to initiate menu changes.