useSlotFills: trigger state update only on array length change

packages/components/src/slot-fill/bubbles-virtually/use-slot-fills.ts

@@ -1,16 +1,42 @@
 /**
  * WordPress dependencies
  */
-import { useContext } from '@wordpress/element';
-import { useObservableValue } from '@wordpress/compose';
+import { useContext, useMemo, useSyncExternalStore } from '@wordpress/element';
+import type { ObservableMap } from '@wordpress/compose';
 
 /**
  * Internal dependencies
  */
 import SlotFillContext from '../context';
 import type { SlotKey } from '../types';
 
+function useObservableValueWithSelector< K, V, S >(
+	map: ObservableMap< K, V >,
+	name: K,
+	selector: ( v: V | undefined ) => S
+) {
+	const subscribe = useMemo(
+		() => ( listener: () => void ) => map.subscribe( name, listener ),
+		[ map, name ]
+	);
+	const getValue = () => selector( map.get( name ) );
+	return useSyncExternalStore( subscribe, getValue, getValue );
+}
+
+function getLength< T >( array: T[] | undefined ) {
+	return array?.length;
+}
+
 export default function useSlotFills( name: SlotKey ) {
 	const registry = useContext( SlotFillContext );
-	return useObservableValue( registry.fills, name );
+	const length = useObservableValueWithSelector(
+		registry.fills,
+		name,
+		getLength
+	);
+	// callers expect an opaque array with length `length`, so create that array
+	const fills = useMemo( () => {
+		return length !== undefined ? Array.from( { length } ) : undefined;
+	}, [ length ] );
+	return fills;
 }