There's actually a separate ListBox control. And I never thought of it at the time, but why does an option turn a ComboBox into a DropDownComboBox, but an option doesn't turn a ListBox into a DropDownListBox? Oh no, it's all coming back to me, because ComboBoxes were added later and rather than change ListBox they added DropDownList functionality to ComboBox.

I also don't see an option in ListBox to make it multi-select, was it all horribly done in the notification handling code?