We'll create fresh WordPress site with WP Dropdown Hierarchical Category UI installed. You have 20 minutes to test the plugin after that site we'll be deleted.
Dropdown hierarchical category in WordPress post write panel or editing screen. It makes the categories or taxonomies list in a better UI which enables show/hide toggle of child categories. It comes with plugin settings page where you can select in what custom post type(s) the dropdown UI will be applied to.
The idea comes from default WordPress hierarchial post categories which looks not good. Imagine that there are a lot of child categories. After selecting child category then publishing or updating a post, the arrangement seems not pretty — the child categories moved away from its parent. This plugin tackles that situation.