Docs: Add test proving scroll is reset across nav

code/e2e-sandbox/addon-docs.spec.ts

@@ -288,4 +288,25 @@ test.describe('addon-docs', () => {
     const root = sbPage.previewRoot();
     await expect(root.getByText('children').first()).toBeVisible();
   });
+
+  test('should reset scroll position between pages', async ({ page }) => {
+    const sbPage = new SbPage(page, expect);
+
+    // Open a long docs page and scroll it down. The preview is rendered inside the iframe, so the
+    // scroll lives on the iframe's documentElement (see WebView.prepareForDocs).
+    await sbPage.navigateToStory('addons/docs/docspage/basic', 'docs');
+
+    const documentElement = sbPage.previewIframe().locator('html');
+    await documentElement.evaluate((el) => {
+      el.scrollTop = 100000;
+    });
+
+    // Confirm the page actually scrolled, otherwise the reset assertion below would be meaningless.
+    await expect.poll(() => documentElement.evaluate((el) => el.scrollTop)).toBeGreaterThan(0);
+
+    // Navigate to a different docs page by clicking its link in the sidebar.
+    await sbPage.navigateToStory('example/button', 'docs');
+
+    await expect.poll(() => documentElement.evaluate((el) => el.scrollTop)).toBe(0);
+  });
 });