fix(nav): hand scroll restoration back to the browser after arrival

app/components/cinematic/useHomeFragmentNavigation.ts

@@ -16,6 +16,17 @@ function focusFragment(target: HTMLElement) {
   target.focus({ preventScroll: true });
 }
 
+/**
+ * A document owns one outstanding homepage arrival. Later mounts are same-document
+ * history, where the browser restores its own position.
+ */
+let arrivalSettled = false;
+
+/** Tests replay several documents inside one jsdom document. */
+export function resetHomeArrivalState() {
+  arrivalSettled = false;
+}
+
 /** Correct only an outstanding arrival; ordinary links and history keep native scrolling. */
 export function useHomeFragmentNavigation() {
   const ready = useRef(false);
@@ -26,12 +37,11 @@ export function useHomeFragmentNavigation() {
       | PerformanceNavigationTiming
       | undefined;
     // A history restoration owns its saved position, not the URL's old fragment.
-    let pending =
-      navigation?.type === "back_forward" ? "" : window.location.hash;
-    let pendingTop =
-      navigation?.type !== "back_forward" && !window.location.hash;
-    if (navigation?.type !== "back_forward")
-      window.history.scrollRestoration = "manual";
+    const restoration = navigation?.type === "back_forward";
+    const arrival = !restoration && !arrivalSettled;
+    let pending = restoration ? "" : window.location.hash;
+    let pendingTop = arrival && !window.location.hash;
+    if (arrival) window.history.scrollRestoration = "manual";
 
     const resolve = () => {
       if (!ready.current) return;
@@ -69,6 +79,7 @@ export function useHomeFragmentNavigation() {
     const cancelArrival = () => {
       pending = "";
       pendingTop = false;
+      window.history.scrollRestoration = "auto";
     };
     const onKeyDown = (event: KeyboardEvent) => {
       if (
@@ -151,5 +162,7 @@ export function useHomeFragmentNavigation() {
   return useCallback(() => {
     ready.current = true;
     resolveArrival.current?.();
+    arrivalSettled = true;
+    window.history.scrollRestoration = "auto";
   }, []);
 }