RenderMenuItems

Subclasses implement this to render the content of the dropdown menu.

They get the whole ControlState, not just the value, because selecting an item sets the value and the text - the menu already knows the label, so there is no reason to work it out a second time.

menuScrollState is the scroll state of the menu itself. This class creates it, because it calls ExposedDropdownMenu, but the subclass loading further pages reads it - hence the parameter. A subclass with the full list of options simply ignores it.