Controls how GuiObject/ZIndex behaves on all descendants of the
LayerCollector.

Enum/ZIndexBehavior/Global sorts all descendants according to the
ZIndex, then breaks ties using the hierarchy order. As a result,
descendants of a GuiObject need to have a ZIndex value that’s at least
as high as the parent, or they will render underneath their parent.

With Enum/ZIndexBehavior/Sibling, children always render above their
parents, and the ZIndex is used to decide the order in which children of a
single UI object will render over each other.