Allow selectors when registering blocks

packages/blocks/src/api/registration.js

@@ -169,6 +169,16 @@ export function unstable__bootstrapServerSideBlockDefinitions( definitions ) {
 				serverSideBlockDefinitions[ blockName ].ancestor =
 					definitions[ blockName ].ancestor;
 			}
+			// The `selectors` prop is not yet included in the server provided
+			// definitions. Polyfill it as well.
+			if (
+				serverSideBlockDefinitions[ blockName ].selectors ===
+					undefined &&
+				definitions[ blockName ].selectors
+			) {
+				serverSideBlockDefinitions[ blockName ].selectors =
+					definitions[ blockName ].selectors;
+			}
 			continue;
 		}
 
@@ -203,6 +213,7 @@ function getBlockSettingsFromMetadata( { textdomain, ...metadata } ) {
 		'attributes',
 		'providesContext',
 		'usesContext',
+		'selectors',
 		'supports',
 		'styles',
 		'example',

packages/blocks/src/api/test/registration.js

@@ -460,6 +460,42 @@ describe( 'blocks', () => {
 			} );
 		} );
 
+		// This can be removed once polyfill adding selectors has been removed.
+		it( 'should apply selectors on the client when not set on the server', () => {
+			const blockName = 'core/test-block-with-selectors';
+			unstable__bootstrapServerSideBlockDefinitions( {
+				[ blockName ]: {
+					category: 'widgets',
+				},
+			} );
+			unstable__bootstrapServerSideBlockDefinitions( {
+				[ blockName ]: {
+					selectors: { root: '.wp-block-custom-selector' },
+					category: 'ignored',
+				},
+			} );
+
+			const blockType = {
+				title: 'block title',
+			};
+			registerBlockType( blockName, blockType );
+			expect( getBlockType( blockName ) ).toEqual( {
+				name: blockName,
+				save: expect.any( Function ),
+				title: 'block title',
+				category: 'widgets',
+				icon: { src: BLOCK_ICON_DEFAULT },
+				attributes: {},
+				providesContext: {},
+				usesContext: [],
+				keywords: [],
+				selectors: { root: '.wp-block-custom-selector' },
+				supports: {},
+				styles: [],
+				variations: [],
+			} );
+		} );
+
 		it( 'should validate the icon', () => {
 			const blockType = {
 				save: noop,