Function panel 

pub fn panel(
    scene: &mut Scene,
    transform: Affine,
    rect: Rect,
    radius: f64,
    theme: &Theme,
)

A floating Panel-role surface (menu, popover, sheet, toast) in one call: the floating material (crate::floating_surface::Material::Floating, the blurred backdrop under a panel-ground tint, opaque where blur is unavailable) with the crate::chrome::panel_border edge on a crisp inset outline — HIG §9.5’s Panel row, so an app never spells the fill + stroke pair itself. Rows inside it use separator with an s3 inset and crate::chrome::selection_tier_on against the panel ground.