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

app/components/cinematic/CinematicHome.test.tsx

@@ -1,5 +1,8 @@
 import { act, fireEvent, render, screen } from "@testing-library/react";
-import { useHomeFragmentNavigation } from "./useHomeFragmentNavigation";
+import {
+  resetHomeArrivalState,
+  useHomeFragmentNavigation,
+} from "./useHomeFragmentNavigation";
 
 function HomeNavigation() {
   const onLayoutReady = useHomeFragmentNavigation();
@@ -23,6 +26,7 @@ function HomeNavigation() {
 const originalScrollIntoView = HTMLElement.prototype.scrollIntoView;
 
 beforeEach(() => {
+  resetHomeArrivalState();
   window.history.replaceState(null, "", "/");
   HTMLElement.prototype.scrollIntoView = jest.fn();
 });
@@ -53,6 +57,26 @@ describe("home fragment navigation", () => {
     expect(restoredPosition).toBe(0);
   });
 
+  it("leaves a same-document return to the position the browser restores", () => {
+    const scrollTo = jest
+      .spyOn(window, "scrollTo")
+      .mockImplementation(() => {});
+    const firstVisit = render(<HomeNavigation />);
+    fireEvent.click(
+      screen.getByRole("button", { name: "Establish notebook layout" }),
+    );
+    firstVisit.unmount();
+    expect(window.history.scrollRestoration).not.toBe("manual");
+
+    scrollTo.mockClear();
+    render(<HomeNavigation />);
+    fireEvent.click(
+      screen.getByRole("button", { name: "Establish notebook layout" }),
+    );
+    expect(scrollTo).not.toHaveBeenCalled();
+    expect(window.history.scrollRestoration).not.toBe("manual");
+  });
+
   it("does not reset the visitor's position after intentional scrolling during startup", () => {
     let position = 0;
     jest