[preact-iso] Router: reset scroll position when navigating forwards

.changeset/rare-seals-burn.md

@@ -0,0 +1,5 @@
+---
+"preact-iso": patch
+---
+
+[preact-iso] Router: reset page scroll position on forward navigations

packages/preact-iso/router.js

@@ -1,17 +1,20 @@
 import { h, createContext, cloneElement } from 'preact';
 import { useContext, useMemo, useReducer, useEffect, useLayoutEffect, useRef } from 'preact/hooks';
 
+let push;
 const UPDATE = (state, url) => {
-	let push = true;
+	push = undefined;
 	if (url && url.type === 'click') {
 		const link = url.target.closest('a[href]');
 		if (!link || link.origin != location.origin || !/^(_?self)?$/i.test(link.target)) return state;
 
+		push = true;
 		url.preventDefault();
 		url = link.href.replace(location.origin, '');
-	} else if (typeof url !== 'string') {
+	} else if (typeof url === 'string') {
+		push = true;
+	} else {
 		url = location.pathname + location.search;
-		push = undefined;
 	}
 
 	if (push === true) history.pushState(null, '', url);
@@ -43,12 +46,13 @@ export const exec = (url, route, matches) => {
 
 export function LocationProvider(props) {
 	const [url, route] = useReducer(UPDATE, location.pathname + location.search);
+	const wasPush = push === true;
 
 	const value = useMemo(() => {
 		const u = new URL(url, location.origin);
 		const path = u.pathname.replace(/(.)\/$/g, '$1');
 		// @ts-ignore-next
-		return { url, path, query: Object.fromEntries(u.searchParams), route };
+		return { url, path, query: Object.fromEntries(u.searchParams), route, wasPush };
 	}, [url]);
 
 	useEffect(() => {
@@ -70,7 +74,7 @@ export function Router(props) {
 
 	const loc = useLocation();
 
-	const { url, path, query } = loc;
+	const { url, path, query, wasPush } = loc;
 
 	const cur = useRef(loc);
 	const prev = useRef();
@@ -115,6 +119,7 @@ export function Router(props) {
 			prev.current = prevChildren.current = pending.current = null;
 			if (props.onLoadEnd) props.onLoadEnd(url);
 			update(0);
+			if (wasPush) scrollTo(0, 0);
 		};
 
 		if (p) {

packages/preact-iso/test/router.test.js

@@ -4,6 +4,8 @@ import { html } from 'htm/preact';
 import { LocationProvider, Router, useLocation } from '../router.js';
 import lazy, { ErrorBoundary } from '../lazy.js';
 
+Object.defineProperty(window, 'scrollTo', { value() {} });
+
 const sleep = ms => new Promise(r => setTimeout(r, ms));
 
 // delayed lazy()
@@ -299,4 +301,50 @@ describe('Router', () => {
 			});
 		}
 	});
+
+	it('should scroll to top when navigating forward', async () => {
+		const scrollTo = jest.spyOn(window, 'scrollTo');
+
+		const Route = jest.fn(() => html`<div style=${{ height: '1000px' }}><a href="/link">link</a></div>`);
+		let loc;
+		render(
+			html`
+				<${LocationProvider}>
+					<${Router}>
+						<${Route} default />
+					<//>
+					<${() => {
+						loc = useLocation();
+					}} />
+				<//>
+			`,
+			scratch
+		);
+
+		await sleep(20);
+
+		expect(scrollTo).not.toHaveBeenCalled();
+		expect(Route).toHaveBeenCalledTimes(1);
+		Route.mockClear();
+
+		loc.route('/programmatic');
+		await sleep(10);
+		expect(loc).toMatchObject({ url: '/programmatic' });
+		expect(scrollTo).toHaveBeenCalledWith(0, 0);
+		expect(scrollTo).toHaveBeenCalledTimes(1);
+		expect(Route).toHaveBeenCalledTimes(1);
+		Route.mockClear();
+		scrollTo.mockClear();
+
+		scratch.querySelector('a').click();
+		await sleep(10);
+		expect(loc).toMatchObject({ url: '/link' });
+		expect(scrollTo).toHaveBeenCalledWith(0, 0);
+		expect(scrollTo).toHaveBeenCalledTimes(1);
+		expect(Route).toHaveBeenCalledTimes(1);
+		Route.mockClear();
+
+		await sleep(10);
+		scrollTo.mockRestore();
+  });
 });