web-player: split player, navigation and accent state into focused modules

web-player/src/lib/state/scroll.ts

@@ -0,0 +1,22 @@
+import { afterNavigate, beforeNavigate } from '$app/navigation';
+import { historyIndex } from './history.svelte';
+
+const scrollPositions = new Map<number, number>();
+
+export function restoreScroll(getScroller: () => HTMLElement | undefined) {
+	let current: number | null = null;
+
+	beforeNavigate(() => {
+		const scroller = getScroller();
+		if (current !== null && scroller) scrollPositions.set(current, scroller.scrollTop);
+	});
+
+	afterNavigate((nav) => {
+		current = historyIndex();
+		const scroller = getScroller();
+		if (!scroller || nav.type === 'enter' || nav.to?.url.hash) return;
+		const saved =
+			nav.type === 'popstate' && current !== null ? scrollPositions.get(current) : undefined;
+		scroller.scrollTop = saved ?? 0;
+	});
+}