Make the scroll-lock test able to fail, and name the tell a broken chain actually shows

e2e-tests/full-flow.spec.ts

@@ -207,6 +207,7 @@ test.describe.serial("authenticated admin journey", () => {
   test("plugin page: a filled page scrolls its table, not the document, and the header stays put", async () => {
     // The region is the viewport less ~185px of chrome, so this leaves ~75px: enough that the 3-row
     // fixture overflows it by half its height, and enough to hold the header a row scrolls under.
+    const original = page.viewportSize();
     await page.setViewportSize({ width: 1280, height: 260 });
     await page.goto("/scheduling/shifts");
 
@@ -235,7 +236,7 @@ test.describe.serial("authenticated admin journey", () => {
     await expect(page.locator("tbody tr").last()).toBeInViewport();
     expect(await headTop(), "the header sticks to a scrollport that moves").toBe(before);
     // The journey shares one page; leaving it short would hand the next test a window it never chose.
-    await page.setViewportSize({ width: 1280, height: 800 });
+    if (original) await page.setViewportSize(original);
   });
 
   test("plugin settings: the screen names the variable that sets each declared key", async () => {

e2e-tests/visual.spec.ts

@@ -57,9 +57,16 @@ for (const [name, path, tail] of [
       if (width <= 860) {
         await page.locator(".hamburger").click(); // the label is the control; the checkbox takes no pointer
         await expect(page.locator("#nav-toggle")).toBeChecked();
+        // The lock is one CSS rule, and whether a key press moves the page with the nav open turns
+        // out to differ by engine — so pin the rule itself, then the behaviour it buys.
+        expect(await page.evaluate(() => getComputedStyle(document.body).overflow)).toBe("hidden");
+        // Both directions: opening the nav may leave the page at either end, and a key press toward
+        // the end it already sits at cannot move it whether the lock holds or not.
         const before = await page.evaluate(() => window.scrollY);
-        await page.keyboard.press("Home");
-        expect(await page.evaluate(() => window.scrollY), "a key press cannot scroll the page while the nav is open").toBe(before);
+        for (const key of ["End", "Home"]) {
+          await page.keyboard.press(key);
+          expect(await page.evaluate(() => window.scrollY), key + " cannot scroll the page while the nav is open").toBe(before);
+        }
       }
     });
   }