Fix useId mismatch due to top level Fragments

package.json

@@ -115,7 +115,7 @@
 		"lint-staged": "^10.5.3",
 		"microbundle": "^0.13.0",
 		"mocha": "^8.2.1",
-		"preact": "^10.5.7",
+		"preact": "^10.11.1",
 		"prettier": "^2.2.1",
 		"sinon": "^9.2.2",
 		"sinon-chai": "^3.5.0",

src/index.js

@@ -239,6 +239,12 @@ function _renderToString(vnode, context, isSvgMode, selectValue, parent) {
 			}
 		}
 
+		// When a component returns a Fragment node we flatten it in core, so we
+		// need to mirror that logic here too
+		let isTopLevelFragment =
+			rendered != null && rendered.type === Fragment && rendered.key == null;
+		rendered = isTopLevelFragment ? rendered.props.children : rendered;
+
 		// Recurse into children before invoking the after-diff hook
 		const str = _renderToString(
 			rendered,

test/render.test.js

@@ -5,7 +5,8 @@ import {
 	useContext,
 	useEffect,
 	useLayoutEffect,
-	useMemo
+	useMemo,
+	useId
 } from 'preact/hooks';
 import { expect } from 'chai';
 import { spy, stub, match } from 'sinon';
@@ -1268,4 +1269,56 @@ describe('render', () => {
 	it('should not render function children', () => {
 		expect(render(<div>{() => {}}</div>)).to.equal('<div></div>');
 	});
+
+	describe('vnode masks (useId)', () => {
+		it('should work with Fragments', () => {
+			const ids = [];
+			function Foo() {
+				const id = useId();
+				ids.push(id);
+				return <p>{id}</p>;
+			}
+
+			function Bar(props) {
+				return props.children;
+			}
+
+			function App() {
+				return (
+					<Bar>
+						<Foo />
+						<Fragment>
+							<Foo />
+						</Fragment>
+					</Bar>
+				);
+			}
+
+			render(<App />);
+			expect(ids[0]).not.to.equal(ids[1]);
+		});
+
+		it('should skip component top level Fragment child', () => {
+			const Wrapper = ({ children }) => <Fragment>{children}</Fragment>;
+
+			function Foo() {
+				const id = useId();
+				return <p>{id}</p>;
+			}
+
+			function App() {
+				const id = useId();
+				return (
+					<div>
+						<p>{id}</p>
+						<Wrapper>
+							<Foo />
+						</Wrapper>
+					</div>
+				);
+			}
+
+			expect(render(<App />)).to.equal('<div><p>P481</p><p>P476951</p></div>');
+		});
+	});
 });