Display parent and child level taxonomy (category) dependent on what content is currently being displayed.

taxonomy_menu_class.php

<?php

function pi_conditions_overview() {
  $conditions = new Tax_List( 'condition' ); // put in name of taxonomy
	$conditions->display_html();
}


class Tax_List {

	private $taxonomy;
	private $post_id;

	public function __construct( $taxonomy ) {
		$this->taxonomy = $taxonomy;
		$this->populate_posts_id();
	}

	// if this is a single post get the ID.
	private function populate_posts_id() {

		if ( ! is_single() ) return;

		$this->post_id = get_the_ID();
	}

	// get just the parent categories.
	private function get_parent_terms() {

		// all terms
		$args = array(
		//	'orderby'		=> 'name',
		//	'order'			=> 'ASC',
			'hide_empty'	=> false,
		//	'exclude'		=> array(),
		//	'exclude_tree'	=> array(),
		//	'include'		=> array(),
		//	'number'		=> '',
		//	'fields'		=> 'all',
		//	'slug'			=> '',
			'parent'		=> 0,
		//	'hierarchical'	=> true,
		//	'child_of'		=> 0,
		//	'get'			=> '',
		//	'name__like'	=> '',
		//	'pad_counts'	=> false,
		//	'offset'		=> '',
		//	'search'		=> '',
		//	'cache_domain'	=> 'core'
		);

		return get_terms( $this->taxonomy, $args );
	}

	/* ===========================================
	 //! Taxonomy checks on current content
	============================================*/

	// are we on a category archive page?
	private function is_this_a_taxonomy( $term_id ) {

		$is_tax = is_tax( $this->taxonomy, $term_id );

		return $is_tax;
	}

	// are we on a child category archive page?
	private function is_this_a_child_taxonomy( $term_id ) {

		return get_queried_object()->parent === $term_id;
	}

	// is_single check if it’s terms correspond in the  taxonomy list.
	// return array of associated term id’s
	private function is_this_a_post( $term_id ) {

		if ( ! $this->post_id ) return;

		$terms = wp_get_post_terms( $this->post_id, $this->taxonomy );

//echo '<pre>' . $this->post_id . '<br/>'.print_r($terms,1).'</pre>';

		if ( ! $terms ) return;

		// each term check if it’s the parent or child of.
		foreach ( $terms as $term )
			if ( $term_id === $term->term_id || $term_id === $term->parent )
				$r[] = $term->term_id;

		return ( $r ? $r : false );
	}

	// determine if this post/cpt post/taxonomy is current, and therefore needs highlighting.
	private function needs_highlight( $term_id, $term_list ) {

		if ( $this->is_this_a_taxonomy( $term_id ) )
			return true;
		elseif ( is_array( $term_list ) && in_array( $term_id, $term_list ) )
			return true;
	}

	/* ===========================================
	//! Views
	============================================*/

	// our output html
	public function display_html() {

		$r = '<ul>';

		foreach ( $this->get_parent_terms() as $term ) {
			$is_parent = $this->is_this_a_taxonomy( $term->term_id );		// is this the parent?
			$is_child = $this->is_this_a_child_taxonomy( $term->term_id );	// is this a child of the term?
			$is_single = $this->is_this_a_post( $term->term_id );			// does this post contain the parent or children of the term?

			// if yes to either we get the child items.
			$children = ( $is_parent || $is_child || $is_single ? get_term_children( $term->term_id, $this->taxonomy ) : null );

			$r .= '<li' . ( $this->needs_highlight( $term->term_id, $is_single ) ? ' class="current"' : null ) . '><a href="' . get_term_link( $term->name, 'condition' ) . '">' . $term->name . '</a>';

				// if this taxonomy has children
				if ( $children ) :

					$r .= '<ul>';

					foreach ( $children as $child ) {
						$childterm = get_term_by( 'id', $child, $this->taxonomy );

						$r .= '<li' . ( $this->needs_highlight( $childterm->term_id, $is_single )  ? ' class="current"' : null ) . '><a href="' . get_term_link( $childterm->name, $this->taxonomy ) . '">' . $childterm->name . '</a></li>';
					}

					$r .= '</ul>';

				endif;

			$r .= '</li>';
		}

		$r .= '</ul>';

		echo $r;
	}
}