Combobox multiple by

[wduandy]

Using the combobox as follows:
<Combobox v-model="selected" multiple :by="key">
I've created a feature to add new items to the selected list.
ComboboxOption exposes selected, and active. 
When i add a new item on the selected list on the parent component, it seems that selected prop is not reactive to this change.
How to make it reactive? i.e. how to select items outside the context of combobox?