diff -r 490d5cc509ed -r cf61fcea0001 wp/wp-admin/includes/class-walker-category-checklist.php
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/wp/wp-admin/includes/class-walker-category-checklist.php	Mon Oct 14 17:39:30 2019 +0200
@@ -0,0 +1,125 @@
+<?php
+/**
+ * Taxonomy API: Walker_Category_Checklist class
+ *
+ * @package WordPress
+ * @subpackage Administration
+ * @since 4.4.0
+ */
+
+/**
+ * Core walker class to output an unordered list of category checkbox input elements.
+ *
+ * @since 2.5.1
+ *
+ * @see Walker
+ * @see wp_category_checklist()
+ * @see wp_terms_checklist()
+ */
+class Walker_Category_Checklist extends Walker {
+	public $tree_type = 'category';
+	public $db_fields = array ('parent' => 'parent', 'id' => 'term_id'); //TODO: decouple this
+
+	/**
+	 * Starts the list before the elements are added.
+	 *
+	 * @see Walker:start_lvl()
+	 *
+	 * @since 2.5.1
+	 *
+	 * @param string $output Used to append additional content (passed by reference).
+	 * @param int    $depth  Depth of category. Used for tab indentation.
+	 * @param array  $args   An array of arguments. @see wp_terms_checklist()
+	 */
+	public function start_lvl( &$output, $depth = 0, $args = array() ) {
+		$indent = str_repeat("\t", $depth);
+		$output .= "$indent<ul class='children'>\n";
+	}
+
+	/**
+	 * Ends the list of after the elements are added.
+	 *
+	 * @see Walker::end_lvl()
+	 *
+	 * @since 2.5.1
+	 *
+	 * @param string $output Used to append additional content (passed by reference).
+	 * @param int    $depth  Depth of category. Used for tab indentation.
+	 * @param array  $args   An array of arguments. @see wp_terms_checklist()
+	 */
+	public function end_lvl( &$output, $depth = 0, $args = array() ) {
+		$indent = str_repeat("\t", $depth);
+		$output .= "$indent</ul>\n";
+	}
+
+	/**
+	 * Start the element output.
+	 *
+	 * @see Walker::start_el()
+	 *
+	 * @since 2.5.1
+	 *
+	 * @param string $output   Used to append additional content (passed by reference).
+	 * @param object $category The current term object.
+	 * @param int    $depth    Depth of the term in reference to parents. Default 0.
+	 * @param array  $args     An array of arguments. @see wp_terms_checklist()
+	 * @param int    $id       ID of the current term.
+	 */
+	public function start_el( &$output, $category, $depth = 0, $args = array(), $id = 0 ) {
+		if ( empty( $args['taxonomy'] ) ) {
+			$taxonomy = 'category';
+		} else {
+			$taxonomy = $args['taxonomy'];
+		}
+
+		if ( $taxonomy == 'category' ) {
+			$name = 'post_category';
+		} else {
+			$name = 'tax_input[' . $taxonomy . ']';
+		}
+
+		$args['popular_cats'] = empty( $args['popular_cats'] ) ? array() : $args['popular_cats'];
+		$class = in_array( $category->term_id, $args['popular_cats'] ) ? ' class="popular-category"' : '';
+
+		$args['selected_cats'] = empty( $args['selected_cats'] ) ? array() : $args['selected_cats'];
+
+		if ( ! empty( $args['list_only'] ) ) {
+			$aria_checked = 'false';
+			$inner_class = 'category';
+
+			if ( in_array( $category->term_id, $args['selected_cats'] ) ) {
+				$inner_class .= ' selected';
+				$aria_checked = 'true';
+			}
+
+			/** This filter is documented in wp-includes/category-template.php */
+			$output .= "\n" . '<li' . $class . '>' .
+				'<div class="' . $inner_class . '" data-term-id=' . $category->term_id .
+				' tabindex="0" role="checkbox" aria-checked="' . $aria_checked . '">' .
+				esc_html( apply_filters( 'the_category', $category->name, '', '' ) ) . '</div>';
+		} else {
+			/** This filter is documented in wp-includes/category-template.php */
+			$output .= "\n<li id='{$taxonomy}-{$category->term_id}'$class>" .
+				'<label class="selectit"><input value="' . $category->term_id . '" type="checkbox" name="'.$name.'[]" id="in-'.$taxonomy.'-' . $category->term_id . '"' .
+				checked( in_array( $category->term_id, $args['selected_cats'] ), true, false ) .
+				disabled( empty( $args['disabled'] ), false, false ) . ' /> ' .
+				esc_html( apply_filters( 'the_category', $category->name, '', '' ) ) . '</label>';
+		}
+	}
+
+	/**
+	 * Ends the element output, if needed.
+	 *
+	 * @see Walker::end_el()
+	 *
+	 * @since 2.5.1
+	 *
+	 * @param string $output   Used to append additional content (passed by reference).
+	 * @param object $category The current term object.
+	 * @param int    $depth    Depth of the term in reference to parents. Default 0.
+	 * @param array  $args     An array of arguments. @see wp_terms_checklist()
+	 */
+	public function end_el( &$output, $category, $depth = 0, $args = array() ) {
+		$output .= "</li>\n";
+	}
+}