Boot: Mount the registered plugins

packages/boot/CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Bug Fixes
 
+-   Mount the plugins registered with `registerPlugin`, so the sidebars, panels and menu items they fill render ([#81754](https://github.com/WordPress/gutenberg/pull/81754)).
 -   Canvas: Show the site's front end where a route names no entity and the theme has no block templates to resolve one from, instead of an editor with nothing open. Was previously handled by the home route alone, leaving `/identity` broken on a classic theme ([#81749](https://github.com/WordPress/gutenberg/pull/81749)).
 -   Canvas: Do not offer to edit a trashed entity from its preview ([#81632](https://github.com/WordPress/gutenberg/pull/81632)).
 -   Translate the click-to-edit label on a previewed canvas, and name the action rather than the gesture ([#81620](https://github.com/WordPress/gutenberg/pull/81620)).

packages/boot/package.json

@@ -58,6 +58,7 @@
 		"@wordpress/keycodes": "file:../keycodes",
 		"@wordpress/lazy-editor": "file:../lazy-editor",
 		"@wordpress/notices": "file:../notices",
+		"@wordpress/plugins": "file:../plugins",
 		"@wordpress/primitives": "file:../primitives",
 		"@wordpress/private-apis": "file:../private-apis",
 		"@wordpress/route": "file:../route",

packages/boot/src/components/plugin-area/index.tsx

@@ -0,0 +1,34 @@
+import { useDispatch } from '@wordpress/data';
+import { __, sprintf } from '@wordpress/i18n';
+import { store as noticesStore } from '@wordpress/notices';
+import { PluginArea as WPPluginArea } from '@wordpress/plugins';
+
+/**
+ * Mounts every plugin registered with `registerPlugin`, so the fills they
+ * render reach their slots.
+ *
+ * Sits at the root rather than with the editor: a plugin's fills mostly target
+ * editor slots, but the component holding them is also where it registers
+ * commands and extends a list, which the routes without a canvas need too.
+ *
+ * @return The mount point. Renders whatever the plugins do, and nothing itself.
+ */
+export default function PluginArea() {
+	const { createErrorNotice } = useDispatch( noticesStore );
+
+	return (
+		<WPPluginArea
+			onError={ ( name ) => {
+				createErrorNotice(
+					sprintf(
+						/* translators: %s: plugin name */
+						__(
+							'The "%s" plugin has encountered an error and cannot be rendered.'
+						),
+						name
+					)
+				);
+			} }
+		/>
+	);
+}

packages/boot/src/components/root/index.tsx

@@ -19,6 +19,7 @@ import Sidebar from '../sidebar';
 import SavePanel from '../save-panel';
 import CanvasRenderer from '../canvas-renderer';
 import ErrorBoundary from '../error-boundary';
+import PluginArea from '../plugin-area';
 import useRouteTitle from '../app/use-route-title';
 import { unlock } from '../../lock-unlock';
 import type { CanvasData } from '../../store/types';
@@ -57,6 +58,7 @@ export default function Root() {
 	return (
 		<SlotFillProvider>
 			<Tooltip.Provider>
+				<PluginArea />
 				<ThemeProvider
 					isRoot
 					color={ { ...themeColors, background: '#f8f8f8' } }

packages/boot/src/components/root/single-page.tsx

@@ -8,6 +8,7 @@ import { ThemeProvider } from '@wordpress/theme';
 import { UnsavedChangesWarning } from '@wordpress/editor';
 import SavePanel from '../save-panel';
 import CanvasRenderer from '../canvas-renderer';
+import PluginArea from '../plugin-area';
 import { unlock } from '../../lock-unlock';
 import type { CanvasData } from '../../store/types';
 import useSyncBodyBackground from './use-sync-body-background';
@@ -39,6 +40,7 @@ export default function RootSinglePage() {
 
 	return (
 		<SlotFillProvider>
+			<PluginArea />
 			<ThemeProvider
 				isRoot
 				color={ { ...themeColors, background: '#f8f8f8' } }

packages/boot/tsconfig.json

@@ -22,6 +22,7 @@
 		{ "path": "../keycodes/tsconfig.build.json" },
 		{ "path": "../lazy-editor" },
 		{ "path": "../notices/tsconfig.build.json" },
+		{ "path": "../plugins" },
 		{ "path": "../primitives" },
 		{ "path": "../private-apis" },
 		{ "path": "../route" },