Make the country and state dropdowns select2 fields when using PMPro and the State Dropdowns Add On.

my_make_country_and_state_select2.php

/**
 * Make country and state dropdowns select2 fields.
 * Requires both the pmpro-state-dropdowns and
 * pmpro-register-helper plugins to be active.
 *
 * Add this to a custom plugin or code snippet.
 * More info here: https://www.paidmembershipspro.com/how-to-add-code-to-wordpress/
 */
function my_make_country_and_state_select2() {
	if ( ! is_admin() ) {
	?>
	<script>
		jQuery(document).ready(function(){
			jQuery('#bcountry').select2();
			jQuery('#bstate').select2();
		});
	</script>
	<?php
	}
}
add_action( 'wp_footer', 'my_make_country_and_state_select2' );

I think some versions of the checkout and/or billing page don't have id attributes on the state and country fields. A different bit of JS to add select2 to those fields may be needed if that's the case. You can select2 all of the select fields in the checkout page form using this JS:

jQuery('form.pmpro-form select').select2();