A MutableListImpl of PopupElement objects that represent an ordered list of pop-up elements. The order of the MutableListImpl is the order that elements are shown in the pop-up window.

For backward compatibility with a webmap or webscene created by clients that don't support the popup elements, a few popup elements are created to match the legacy popup style when reading a popup definition without popup elements:

Vice versa, if the user updates the popup elements, the legacy popup style is updated during the PopupDefinition serialization, in order to match as much as possible the popup elements, so that older clients can display a popup that looks similar.