fix: Back button sets view to where you left page (#7370)

Co-authored-by: Michael Genson <genson.michael@gmail.com>

frontend/composables/use-scroll-position.ts

@@ -0,0 +1,65 @@
+const scrollPositions = new Map<string, number>();
+const pagePositions = new Map<string, number>();
+
+export function useScrollPosition() {
+  const router = useRouter();
+
+  let observer: MutationObserver | null = null;
+  let timeout: ReturnType<typeof setTimeout> | null = null;
+  let fallback: ReturnType<typeof setTimeout> | null = null;
+
+  function savePosition(path: string, page: number) {
+    scrollPositions.set(path, document.documentElement.scrollTop);
+    pagePositions.set(path, page);
+  }
+
+  function getSavedPage(path: string): number | undefined {
+    return pagePositions.get(path);
+  }
+
+  function restorePosition(path: string) {
+    const savedPosition = scrollPositions.get(path);
+    if (!savedPosition) return;
+
+    observer?.disconnect();
+    if (timeout) clearTimeout(timeout);
+    if (fallback) clearTimeout(fallback);
+
+    fallback = setTimeout(() => {
+      if (timeout) clearTimeout(timeout);
+      observer?.disconnect();
+      document.documentElement.scrollTop = savedPosition;
+    }, 500);
+
+    observer = new MutationObserver(() => {
+      if (timeout) clearTimeout(timeout);
+      timeout = setTimeout(() => {
+        if (fallback) clearTimeout(fallback);
+        observer?.disconnect();
+        document.documentElement.scrollTop = savedPosition;
+      }, 100);
+    });
+
+    observer.observe(document.body, {
+      childList: true,
+      subtree: true,
+    });
+  }
+
+  const unregisterBefore = router.beforeEach((to, from) => {
+    scrollPositions.set(from.path, document.documentElement.scrollTop);
+  });
+
+  onUnmounted(() => {
+    unregisterBefore();
+    observer?.disconnect();
+    if (timeout) clearTimeout(timeout);
+    if (fallback) clearTimeout(fallback);
+  });
+
+  return {
+    savePosition,
+    getSavedPage,
+    restorePosition,
+  };
+}