fix(#8625): smooth scrolling in SPA mode on iOS

packages/astro/src/transitions/router.ts

@@ -627,7 +627,7 @@ function onPopState(ev: PopStateEvent) {
 
 // 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 onScroll = () => {
+const onScrollEnd = () => {
 	updateScrollPosition({ scrollX, scrollY });
 };
 
@@ -637,8 +637,36 @@ if (inBrowser) {
 		originalLocation = new URL(location.href);
 		addEventListener('popstate', onPopState);
 		addEventListener('load', onPageLoad);
-		if ('onscrollend' in window) addEventListener('scrollend', onScroll);
-		else addEventListener('scroll', throttle(onScroll, 350), { passive: true });
+		if ('onscrollend' in window) addEventListener('scrollend', onScrollEnd);
+		else {
+			// Keep track of state between intervals
+			let intervalId: number | undefined, lastY = scrollY, lastX = scrollX, lastIndex = history.state?.index;
+			const scrollInterval = () => {
+				// Check the index if a popstate event fired between intervals
+				if (lastIndex !== history.state?.index) {
+					// Cancel the interval because we're probably not scrolling
+					clearInterval(intervalId);
+					intervalId = undefined;
+					// Update index and return
+					lastIndex = history.state?.index;
+					return;
+				}
+				// Check if the user stopped scrolling *and* if the positions need to be updated
+				if (
+					lastY === scrollY && lastX === scrollX
+					&& (lastY !== history.state.scrollY || lastX !== history.state.scrollX)
+				) {
+					// Cancel the interval and update scroll positions
+					clearInterval(intervalId);
+					intervalId = undefined;
+					onScrollEnd();
+				}
+				// Update variables for the next interval
+				lastIndex = history.state?.index, lastY = scrollY, lastX = scrollX;
+			}
+			// We can't know when or how often scroll events fire, so we'll just use them to start intervals
+			addEventListener("scroll", () => intervalId ??= window.setInterval(scrollInterval, 200), { passive: true });
+		};
 	}
 	for (const script of document.scripts) {
 		script.dataset.astroExec = '';