Restore horizontal scroll position on history navigation (view transitions) (#8505)

4 files changed, 74 insertions, 11 deletions

diff --git a/.changeset/polite-ravens-serve.md b/.changeset/polite-ravens-serve.md
new file mode 100644
index 000000000..0f5d28c92
--- /dev/null
+++ b/.changeset/polite-ravens-serve.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Restore horizontal scroll position on history navigation (view transitions)
diff --git a/packages/astro/components/ViewTransitions.astro b/packages/astro/components/ViewTransitions.astro
index 4d615188d..6ae8f3592 100644
--- a/packages/astro/components/ViewTransitions.astro
+++ b/packages/astro/components/ViewTransitions.astro
@@ -15,6 +15,7 @@ const { fallback = 'animate' } = Astro.props as Props;
 	type Direction = 'forward' | 'back';
 	type State = {
 		index: number;
+		scrollX: number;
 		scrollY: number;
 	};
 	type Events = 'astro:page-load' | 'astro:after-swap';
@@ -37,9 +38,9 @@ const { fallback = 'animate' } = Astro.props as Props;
 		// we reloaded a page with history state
 		// (e.g. history navigation from non-transition page or browser reload)
 		currentHistoryIndex = history.state.index;
-		scrollTo({ left: 0, top: history.state.scrollY });
+		scrollTo({ left: history.state.scrollX, top: history.state.scrollY });
 	} else if (transitionEnabledOnThisPage()) {
-		history.replaceState({ index: currentHistoryIndex, scrollY }, '');
+		history.replaceState({ index: currentHistoryIndex, scrollX, scrollY }, '');
 	}
 	const throttle = (cb: (...args: any[]) => any, delay: number) => {
 		let wait = false;
@@ -208,17 +209,29 @@ const { fallback = 'animate' } = Astro.props as Props;
 			// Chromium based browsers (Chrome, Edge, Opera, ...)
 			scrollTo({ left: 0, top: 0, behavior: 'instant' });
 
+			let initialScrollX = 0;
 			let initialScrollY = 0;
 			if (!state && loc.hash) {
 				const id = decodeURIComponent(loc.hash.slice(1));
 				const elem = document.getElementById(id);
 				// prefer scrollIntoView() over scrollTo() because it takes scroll-padding into account
-				elem && (initialScrollY = elem.offsetTop) && elem.scrollIntoView();
-			} else if (state && state.scrollY !== 0) {
-				scrollTo(0, state.scrollY); // usings default scrollBehavior
+				if (elem) {
+					elem.scrollIntoView();
+					initialScrollX = Math.max(
+						0,
+						elem.offsetLeft + elem.offsetWidth - document.documentElement.clientWidth
+					);
+					initialScrollY = elem.offsetTop;
+				}
+			} else if (state) {
+				scrollTo(state.scrollX, state.scrollY); // usings default scrollBehavior
 			}
 			!state &&
-				history.pushState({ index: ++currentHistoryIndex, scrollY: initialScrollY }, '', loc.href);
+				history.pushState(
+					{ index: ++currentHistoryIndex, scrollX: initialScrollX, scrollY: initialScrollY },
+					'',
+					loc.href
+				);
 			triggerEvent('astro:after-swap');
 		};
 
@@ -280,7 +293,7 @@ const { fallback = 'animate' } = Astro.props as Props;
 		}
 
 		// Now we are sure that we will push state, and it is time to create a state if it is still missing.
-		!state && history.replaceState({ index: currentHistoryIndex, scrollY }, '');
+		!state && history.replaceState({ index: currentHistoryIndex, scrollX, scrollY }, '');
 
 		document.documentElement.dataset.astroTransition = dir;
 		if (supportsViewTransitions) {
@@ -357,8 +370,11 @@ const { fallback = 'animate' } = Astro.props as Props;
 					ev.preventDefault();
 					// push state on the first navigation but not if we were here already
 					if (location.hash) {
-						history.replaceState({ index: currentHistoryIndex, scrollY: -(scrollY + 1) }, '');
-						const newState: State = { index: ++currentHistoryIndex, scrollY: 0 };
+						history.replaceState(
+							{ index: currentHistoryIndex, scrollX, scrollY: -(scrollY + 1) },
+							''
+						);
+						const newState: State = { index: ++currentHistoryIndex, scrollX: 0, scrollY: 0 };
 						history.pushState(newState, '', link.href);
 					}
 					scrollTo({ left: 0, top: 0, behavior: 'instant' });
@@ -404,7 +420,7 @@ const { fallback = 'animate' } = Astro.props as Props;
 			const direction: Direction = nextIndex > currentHistoryIndex ? 'forward' : 'back';
 			currentHistoryIndex = nextIndex;
 			if (state.scrollY < 0) {
-				scrollTo(0, -(state.scrollY + 1));
+				scrollTo(state.scrollX, -(state.scrollY + 1));
 			} else {
 				navigate(direction, new URL(location.href), state);
 			}
@@ -432,7 +448,7 @@ const { fallback = 'animate' } = Astro.props as Props;
 		// There's not a good way to record scroll position before a back button.
 		// So the way we do it is by listening to scrollend if supported, and if not continuously record the scroll position.
 		const updateState = () => {
-			persistState({ ...history.state, scrollY });
+			persistState({ ...history.state, scrollX, scrollY });
 		};
 
 		if ('onscrollend' in window) addEventListener('scrollend', updateState);
diff --git a/packages/astro/e2e/fixtures/view-transitions/src/pages/wide-page.astro b/packages/astro/e2e/fixtures/view-transitions/src/pages/wide-page.astro
new file mode 100644
index 000000000..4862122c0
--- /dev/null
+++ b/packages/astro/e2e/fixtures/view-transitions/src/pages/wide-page.astro
@@ -0,0 +1,12 @@
+---
+import Layout from '../components/Layout.astro';
+---
+<Layout>
+	<a id="click-right" href="#click-one">go right</a>
+	<article id="widepage">
+		<div style="width:300vw; text-align: center">
+			<a id="click-top" href="#click-right">go to top</a> |
+			<a id="click-one" href="/one">go to 1</a>
+		</div>
+	</article>
+</Layout>
diff --git a/packages/astro/e2e/view-transitions.test.js b/packages/astro/e2e/view-transitions.test.js
index 64041ab05..6421b5fc3 100644
--- a/packages/astro/e2e/view-transitions.test.js
+++ b/packages/astro/e2e/view-transitions.test.js
@@ -585,4 +585,34 @@ test.describe('View Transitions', () => {
 		styles = await page.locator('style').all();
 		expect(styles.length).toEqual(totalExpectedStyles, 'style count has not changed');
 	});
+
+	test('Horizontal scroll position restored on back button', async ({ page, astro }) => {
+		await page.goto(astro.resolveUrl('/wide-page'));
+		let article = page.locator('#widepage');
+		await expect(article, 'should have script content').toBeVisible('exists');
+
+		let locator = page.locator('#click-one');
+		await expect(locator).not.toBeInViewport();
+
+		await page.click('#click-right');
+		locator = page.locator('#click-one');
+		await expect(locator).toBeInViewport();
+		locator = page.locator('#click-top');
+		await expect(locator).toBeInViewport();
+
+		await page.click('#click-one');
+		let p = page.locator('#one');
+		await expect(p, 'should have content').toHaveText('Page 1');
+
+		await page.goBack();
+		locator = page.locator('#click-one');
+		await expect(locator).toBeInViewport();
+
+		locator = page.locator('#click-top');
+		await expect(locator).toBeInViewport();
+
+		await page.click('#click-top');
+		locator = page.locator('#click-one');
+		await expect(locator).not.toBeInViewport();
+	});
 });