Blocks: Ensure that metadata registered on the server for core block is preserved on the client

packages/blocks/src/api/registration.js

@@ -160,6 +160,11 @@ export const serverSideBlockDefinitions = {};
 // eslint-disable-next-line camelcase
 export function unstable__bootstrapServerSideBlockDefinitions( definitions ) {
 	for ( const blockName of Object.keys( definitions ) ) {
+		// Don't overwrite if already set. It covers the case when metadata
+		// was initialized from the server.
+		if ( serverSideBlockDefinitions[ blockName ] ) {
+			continue;
+		}
 		serverSideBlockDefinitions[ blockName ] = mapKeys(
 			pickBy( definitions[ blockName ], ( value ) => ! isNil( value ) ),
 			( value, key ) => camelCase( key )

packages/e2e-tests/plugins/register-block-type-hooks.php

@@ -0,0 +1,28 @@
+<?php
+/**
+ * Plugin Name: Gutenberg Test Register Block Type Hooks
+ * Plugin URI: https://github.com/WordPress/gutenberg
+ * Author: Gutenberg Team
+ *
+ * @package gutenberg-test-register-block-type-hooks
+ */
+
+/**
+ * Changes the category for the paragraph block.
+ *
+ * @param array $metadata Array of metadata for registering a block type.
+ *
+ * @return array Filtered metadata for registering a block type.
+ */
+function gutenberg_test_block_type_metadata( $metadata ) {
+	if ( 'core/paragraph' !== $metadata['name'] ) {
+		return $metadata;
+	}
+
+	return array_merge(
+		$metadata,
+		array( 'category' => 'widgets' )
+	);
+}
+
+add_filter( 'block_type_metadata', 'gutenberg_test_block_type_metadata' );

packages/e2e-tests/specs/editor/plugins/register-block-type-hooks.test.js

@@ -0,0 +1,32 @@
+/**
+ * WordPress dependencies
+ */
+import {
+	activatePlugin,
+	createNewPost,
+	deactivatePlugin,
+	openGlobalBlockInserter,
+} from '@wordpress/e2e-test-utils';
+
+describe( 'Register block type hooks', () => {
+	beforeEach( async () => {
+		await activatePlugin( 'gutenberg-test-register-block-type-hooks' );
+		await createNewPost();
+	} );
+
+	afterEach( async () => {
+		await deactivatePlugin( 'gutenberg-test-register-block-type-hooks' );
+	} );
+
+	it( 'has a custom category for Paragraph block', async () => {
+		await openGlobalBlockInserter();
+
+		const widgetsCategory = await page.$(
+			'.block-editor-block-types-list[aria-label="Widgets"]'
+		);
+
+		expect(
+			await widgetsCategory.$( '.editor-block-list-item-paragraph' )
+		).toBeDefined();
+	} );
+} );