From acf652fc1d5db166231e87e22d0d50444f5556d8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jacob=20Gro=C3=9F?= <kurtextrem@gmail.com>
Date: Fri, 25 Aug 2023 14:43:52 +0200
Subject: [PATCH] feat(ViewTransitions): use `scrollend` instead of `scroll`
 where supported (#8156)

* feat(ViewTransitions): use `scrollend` instead of `scroll` where supported

The [scrollend](https://developer.chrome.com/blog/scrollend-a-new-javascript-event/#event-details) mechanism seems like a better way to record the scroll position compared to throttling, so we could use it whenever a browser supports it.

Additionally, I've removed the `{passive}` flag from the `scroll` event, as it does nothing ([source](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener?retiredLocale=de#improving_scrolling_performance_with_passive_listeners:~:text=You%20don%27t%20need%20to%20worry%20about%20the%20value%20of%20passive%20for%20the%20basic%20scroll%20event.%20Since%20it%20can%27t%20be%20canceled%2C%20event%20listeners%20can%27t%20block%20page%20rendering%20anyway.)).

* Create long-chefs-jump.md

* fix typo / update comment
---
 .changeset/long-chefs-jump.md                 |  5 +++++
 .../astro/components/ViewTransitions.astro    | 22 +++++++++----------
 2 files changed, 15 insertions(+), 12 deletions(-)
 create mode 100644 .changeset/long-chefs-jump.md

diff --git a/.changeset/long-chefs-jump.md b/.changeset/long-chefs-jump.md
new file mode 100644
index 000000000000..ed8f47614bfb
--- /dev/null
+++ b/.changeset/long-chefs-jump.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+The scrollend mechanism is a better way to record the scroll position compared to throttling, so we now use it whenever a browser supports it.
\ No newline at end of file
diff --git a/packages/astro/components/ViewTransitions.astro b/packages/astro/components/ViewTransitions.astro
index 42e8deb74b74..7fa33676825a 100644
--- a/packages/astro/components/ViewTransitions.astro
+++ b/packages/astro/components/ViewTransitions.astro
@@ -385,17 +385,15 @@ const { fallback = 'animate' } = Astro.props as Props;
 		});
 		addEventListener('load', onPageLoad);
 		// There's not a good way to record scroll position before a back button.
-		// So the way we do it is by listening to scroll and just continuously recording it.
-		addEventListener(
-			'scroll',
-			throttle(() => {
-				// only updste history entries that are managed by us
-				// leave other entries alone and do not accidently add state.
-				if (history.state) {
-					persistState({ ...history.state, scrollY });
-				}
-			}, 300),
-			{ passive: true }
-		);
+		// So the way we do it is by listening to scrollend if supported, and if not continuously record the scroll position.
+    const updateState = () => {
+			// only update history entries that are managed by us
+			// leave other entries alone and do not accidently add state.
+			if (history.state) {
+				persistState({ ...history.state, scrollY });
+			}
+		}
+		if ('onscrollend' in window) addEventListener('scrollend', updateState);
+		else addEventListener('scroll', throttle(updateState, 300));
 	}
 </script>