When multiple CSS classes are added to a menu item, the nav_menu_link_attributes
filter should be called with $item->classes set to an array of CSS class names.
When previewing in the Customizer, however, a single string was being passed to
$item->classes because WP_Customize_Nav_Menu_Item_Setting::preview() bypasses
wp_update_nav_menu_item() and instead uses filter_wp_get_nav_menu_items().
The fix is to make filter_wp_get_nav_menu_items() match what
wp_update_nav_menu_item() does and split the string into an array.
Fixes#43113.
Props dlh.
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