submenu property

Child items shown when this item is activated inside a GlassMenu.

When non-null, activating the item (tap, slide-and-release, or GlassMenuController.endGlide) does not call onTap or close the menu. Instead a card opens over the parent, which remains visible but recedes and dims. The card's header repeats this item's icon and title, in bold, with a downward chevron. Activating the header collapses the card. Submenus may nest; only the frontmost card's actions are interactive.

When trailing is null a submenu item shows a chevron.