Function
DropdownMenuContent already wraps itself in one, so callers rarely
need this directly. It matters twice: a menu opened from inside a clipped or
transformed ancestor would otherwise be cut off by it, and in tests portalled
content is not inside the container returned by RTL's render, so
assertions must go through screen or document.
Portal that lifts the menu out of the DOM position where it was declared and mounts it at the document root.