Function z_index 

pub fn z_index(child: impl IntoUiNode, index: impl IntoVar<ZIndex>) -> UiNode

P Defines the render order of a widget in a layout panel.

When set the widget will still update and layout according to their logical position in the list but they will render according to the order defined by the ZIndex value.

An error is logged on init if the widget is not a direct child of a Z-sorting panel.