diff -r 3d4e9c994f10 -r a86126ab1dd4 wp/wp-admin/includes/class-walker-category-checklist.php
--- a/wp/wp-admin/includes/class-walker-category-checklist.php	Tue Oct 22 16:11:46 2019 +0200
+++ b/wp/wp-admin/includes/class-walker-category-checklist.php	Tue Dec 15 13:49:49 2020 +0100
@@ -21,7 +21,7 @@
 	public $db_fields = array(
 		'parent' => 'parent',
 		'id'     => 'term_id',
-	); //TODO: decouple this
+	); // TODO: Decouple this.
 
 	/**
 	 * Starts the list before the elements are added.
@@ -75,37 +75,41 @@
 			$taxonomy = $args['taxonomy'];
 		}
 
-		if ( $taxonomy == 'category' ) {
+		if ( 'category' === $taxonomy ) {
 			$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['popular_cats'] = ! empty( $args['popular_cats'] ) ? array_map( 'intval', $args['popular_cats'] ) : array();
 
-		$args['selected_cats'] = empty( $args['selected_cats'] ) ? array() : $args['selected_cats'];
+		$class = in_array( $category->term_id, $args['popular_cats'], true ) ? ' class="popular-category"' : '';
+
+		$args['selected_cats'] = ! empty( $args['selected_cats'] ) ? array_map( 'intval', $args['selected_cats'] ) : array();
 
 		if ( ! empty( $args['list_only'] ) ) {
 			$aria_checked = 'false';
 			$inner_class  = 'category';
 
-			if ( in_array( $category->term_id, $args['selected_cats'] ) ) {
+			if ( in_array( $category->term_id, $args['selected_cats'], true ) ) {
 				$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 . '">' .
+				/** This filter is documented in wp-includes/category-template.php */
 				esc_html( apply_filters( 'the_category', $category->name, '', '' ) ) . '</div>';
 		} else {
-			/** This filter is documented in wp-includes/category-template.php */
+			$is_selected = in_array( $category->term_id, $args['selected_cats'], true );
+			$is_disabled = ! empty( $args['disabled'] );
+
 			$output .= "\n<li id='{$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 ) . ' /> ' .
+				checked( $is_selected, true, false ) .
+				disabled( $is_disabled, true, false ) . ' /> ' .
+				/** This filter is documented in wp-includes/category-template.php */
 				esc_html( apply_filters( 'the_category', $category->name, '', '' ) ) . '</label>';
 		}
 	}