This is rather confusing as what gets typed do not show up. Probably shifting to an input with <select> options might help ? Need to check what OOJS component are used here though.
|mediawiki/core : master||Change DropdownWidgets to ComboBoxInputWidgets|
What you type doesn't show up in a normal HTML <select> either, at least not in any browser I've ever used. The main difference I see is that a Firefox <select> immediately "selects" the currently highlighted value in the widget (no way to cancel if you change your mind) while the ooui dropdown needs an option to be explicitly selected.
Allowing people to enter random values and rejecting them if they aren't what's in the dropdown seems like poor UI compared to the existing use of dropdown widgets that don't allow selecting invalid values in the first place. Unless Design thinks otherwise, I'm inclined to decline this task.