custom select field not searchable when accessed by keyboard

whenever a select2 widget is implemented and the option list exceeds the visible list, a search box is provided at the top allowing the user to filter the list. this is most useful when the option list is long.

for select-list custom fields, we've found that if the field is accessed via the mouse, that search filter is present and functional. but if the user is tabbing through fields, gets to a select field and begins typing – there is no filter.

interestingly, this does work for some core fields. for example if you tab to the state field and begin typing it will dynamically filter. so I suspect it is an issue of a config setting in the select2 implementation to enable the filter when accessed in this way.