I've created a menu icon which has a mouseover set to reveal a pop-up layer. On the popup layer, I've inserted a vertical navbar. This works fine. However, some of the navbar items have sub-menus. Those sub-menus display as expected, but when the mouse cursor is moved over the sub-menus, the pop-up layer hides, so the menu disappears. How can I make the popup layer stay visible whilst the user is interacting with the menu items?