In most cases the user will not touch the widget, so we should probably wait for them to interact with it before infusing.

I think we could do this more easily by making DropdownInputWidget lazy-initialize the menu.

In T203838#4568159, @matmarex wrote:

Lazy-infusing a widget on click might indeed lead to weird interactions, but it should be perfectly possible to infuse it as normal, but not generate all the MenuOptionWidgets until the dropdown handle is clicked. You could do that in a subclass of DropdownInputWidget, but really vanilla DropdownInputWidget could work that way too…

Something very similar is already implemented in mw.rcfilters.ui.MenuSelectWidget (which fills itself from the toggle() method), surely it will also work for DropdownInputWidget, which is much simpler.