diff -r c7c34916027a -r 177826044cd9 wp/wp-includes/blocks/categories.php
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/wp/wp-includes/blocks/categories.php	Mon Oct 14 18:28:13 2019 +0200
@@ -0,0 +1,102 @@
+<?php
+/**
+ * Server-side rendering of the `core/categories` block.
+ *
+ * @package WordPress
+ */
+
+/**
+ * Renders the `core/categories` block on server.
+ *
+ * @param array $attributes The block attributes.
+ *
+ * @return string Returns the categories list/dropdown markup.
+ */
+function render_block_core_categories( $attributes ) {
+	static $block_id = 0;
+	$block_id++;
+
+	$args = array(
+		'echo'         => false,
+		'hierarchical' => ! empty( $attributes['showHierarchy'] ),
+		'orderby'      => 'name',
+		'show_count'   => ! empty( $attributes['showPostCounts'] ),
+		'title_li'     => '',
+	);
+
+	if ( ! empty( $attributes['displayAsDropdown'] ) ) {
+		$id                       = 'wp-block-categories-' . $block_id;
+		$args['id']               = $id;
+		$args['show_option_none'] = __( 'Select Category' );
+		$wrapper_markup           = '<div class="%1$s">%2$s</div>';
+		$items_markup             = wp_dropdown_categories( $args );
+		$type                     = 'dropdown';
+
+		if ( ! is_admin() ) {
+			$wrapper_markup .= build_dropdown_script_block_core_categories( $id );
+		}
+	} else {
+		$wrapper_markup = '<ul class="%1$s">%2$s</ul>';
+		$items_markup   = wp_list_categories( $args );
+		$type           = 'list';
+	}
+
+	$class = "wp-block-categories wp-block-categories-{$type}";
+
+	if ( isset( $attributes['align'] ) ) {
+		$class .= " align{$attributes['align']}";
+	}
+
+	if ( isset( $attributes['className'] ) ) {
+		$class .= " {$attributes['className']}";
+	}
+
+	$block_content = sprintf(
+		$wrapper_markup,
+		esc_attr( $class ),
+		$items_markup
+	);
+
+	return $block_content;
+}
+
+/**
+ * Generates the inline script for a categories dropdown field.
+ *
+ * @param string $dropdown_id ID of the dropdown field.
+ *
+ * @return string Returns the dropdown onChange redirection script.
+ */
+function build_dropdown_script_block_core_categories( $dropdown_id ) {
+	ob_start();
+	?>
+	<script type='text/javascript'>
+	/* <![CDATA[ */
+	( function() {
+		var dropdown = document.getElementById( '<?php echo esc_js( $dropdown_id ); ?>' );
+		function onCatChange() {
+			if ( dropdown.options[ dropdown.selectedIndex ].value > 0 ) {
+				location.href = "<?php echo home_url(); ?>/?cat=" + dropdown.options[ dropdown.selectedIndex ].value;
+			}
+		}
+		dropdown.onchange = onCatChange;
+	})();
+	/* ]]> */
+	</script>
+	<?php
+	return ob_get_clean();
+}
+
+/**
+ * Registers the `core/categories` block on server.
+ */
+function register_block_core_categories() {
+	register_block_type(
+		'core/categories',
+		array(
+			'render_callback' => 'render_block_core_categories',
+		)
+	);
+}
+
+add_action( 'init', 'register_block_core_categories' );