Function menu_panel 

pub fn menu_panel(
    ui: &mut Ui<'_>,
    label: &str,
    spec: NodeSpec,
    items: &[MenuItem],
) -> Key

Builds the rows of one menu into spec, keyed under label, and reports the keys they took. The one place a menu’s rows are drawn: both menus kui has are this function with a different container.

A row with a submenu (MenuItem::submenu) is drawn with a chevron. In the core’s own menus — the context menu and the drawn bar’s — its menu opens beside it, as another of these panels, when the pointer rests on it, it is clicked, or the keyboard opens it (Enter, the Right arrow); an app drawing this panel itself gets the chevron and opens nothing, since the open submenus are the core’s state.