Currently, text selection marks (text selection redactions, highlight annotations) do not have the ability to have their z-order changed. so text selection marks will always appear behind other types of marks. Text selection marks will also always order themselves by putting the least recently changed/created text selection marks on top.
In the use case where the user wants to be able to sort the newest marks on top, not having the ability to control the z-order of text selection marks makes this not possible because they always appear on the bottom.
William Bell
