First pass at forking the add page command depending on context.

packages/core-commands/src/admin-navigation-commands.js

@@ -1,9 +1,76 @@
 /**
  * WordPress dependencies
  */
-import { useCommand } from '@wordpress/commands';
+import { useCommand, useCommandLoader } from '@wordpress/commands';
 import { __ } from '@wordpress/i18n';
 import { plus } from '@wordpress/icons';
+import { getPath } from '@wordpress/url';
+import { store as coreStore } from '@wordpress/core-data';
+import { useDispatch, useSelect } from '@wordpress/data';
+import { useMemo, useCallback } from '@wordpress/element';
+
+function useAddNewPageCommand() {
+	const isSiteEditor = getPath( window.location.href )?.includes(
+		'site-editor.php'
+	);
+	const { userCanCreatePages } = useSelect( ( select ) => {
+		const { canUser } = select( coreStore );
+		return {
+			userCanCreatePages: canUser( 'create', {
+				kind: 'postType',
+				name: 'page',
+			} ),
+		};
+	}, [] );
+	const { saveEntityRecord } = useDispatch( coreStore );
+	/**
+	 * Creates a Post entity.
+	 *
+	 * @param {Object} options parameters for the post being created. These mirror those used on 3rd param of saveEntityRecord.
+	 * @return {Object} the post type object that was created.
+	 */
+	const createPageEntity = useCallback(
+		( options ) => {
+			if ( ! userCanCreatePages ) {
+				return Promise.reject( {
+					message: __(
+						'You do not have permission to create Pages.'
+					),
+				} );
+			}
+			return saveEntityRecord( 'postType', 'page', options );
+		},
+		[ saveEntityRecord, userCanCreatePages ]
+	);
+
+	const commands = useMemo( () => {
+		if ( ! userCanCreatePages ) {
+			return [];
+		}
+		return [
+			{
+				name: 'core/add-new-page',
+				label: __( 'Add new page' ),
+				icon: plus,
+				callback: async () => {
+					if ( isSiteEditor ) {
+						const page = await createPageEntity( {
+							status: 'draft',
+						} );
+						document.location.href = `/wp-admin/site-editor.php?postId=${ page.id }&postType=page&canvas=edit`;
+						return;
+					}
+					document.location.href = 'post-new.php?post_type=page';
+				},
+			},
+		];
+	}, [ createPageEntity, isSiteEditor, userCanCreatePages ] );
+
+	return {
+		isLoading: false,
+		commands,
+	};
+}
 
 export function useAdminNavigationCommands() {
 	useCommand( {
@@ -14,12 +81,9 @@ export function useAdminNavigationCommands() {
 			document.location.href = 'post-new.php';
 		},
 	} );
-	useCommand( {
+
+	useCommandLoader( {
 		name: 'core/add-new-page',
-		label: __( 'Add new page' ),
-		icon: plus,
-		callback: () => {
-			document.location.href = 'post-new.php?post_type=page';
-		},
+		hook: useAddNewPageCommand,
 	} );
 }