fix(boot): a stylesheet lost on the way is asked for again

A lost stylesheet left the page unstyled until a reload, and every e2e step that finds elements by
testid still passed on it. Only axe noticed: the one-off target-size failure on eight scattered
controls. Losing the main stylesheet on purpose gives those same eight.

frontend/e2e/stylesheet.spec.ts

@@ -0,0 +1,60 @@
+import { test, expect, type Page } from "@playwright/test";
+import { openApp } from "./helpers";
+
+// A request can be lost on the way on a loaded machine or a poor line. A lost stylesheet leaves the
+// whole page unstyled, and every test that finds its elements by testid still passes on it: only an
+// axe run ever noticed (8 controls under the target size). So this looks at the styles themselves.
+
+/** Lose the first request for every stylesheet of the page; the ones after it go through. */
+async function loseEachStylesheetOnce(page: Page): Promise<string[]> {
+  const lost: string[] = [];
+  await page.route(/\/assets\/[^/]+\.css/, async (route) => {
+    const url = route.request().url();
+    if (lost.includes(url)) return route.continue();
+    lost.push(url);
+    return route.abort("failed");
+  });
+  return lost;
+}
+
+/** Every stylesheet link of the page holds rules (a lost one still has a `sheet`, whose rules throw
+ *  on reading), and the page's own tokens are in force: `--bg` comes from the stylesheet alone, while
+ *  `--font-scale` would prove nothing, the app also sets it from script. */
+async function expectStyled(page: Page): Promise<void> {
+  await expect
+    .poll(() =>
+      page.evaluate(() => {
+        const hasRules = (l: HTMLLinkElement) => {
+          try {
+            return (l.sheet?.cssRules.length ?? 0) > 0;
+          } catch {
+            return false;
+          }
+        };
+        const links = [...document.querySelectorAll<HTMLLinkElement>('link[rel="stylesheet"]')];
+        const root = getComputedStyle(document.documentElement);
+        return {
+          links: links.length > 0,
+          rules: links.every(hasRules),
+          tokens: root.getPropertyValue("--bg").trim() !== "",
+        };
+      }),
+    )
+    .toEqual({ links: true, rules: true, tokens: true });
+}
+
+test("the app gets back a stylesheet lost on the way", async ({ page }) => {
+  const lost = await loseEachStylesheetOnce(page);
+  await openApp(page);
+  expect(lost.length).toBeGreaterThan(0);
+  await expectStyled(page);
+});
+
+test("a public /watch link gets back a stylesheet lost on the way", async ({ page }) => {
+  const lost = await loseEachStylesheetOnce(page);
+  // No such link: the page says so, and it is the same entry with the same stylesheet.
+  await page.goto("/watch/no-such-token");
+  await expect(page.locator("#root")).not.toBeEmpty();
+  expect(lost.length).toBeGreaterThan(0);
+  await expectStyled(page);
+});

frontend/src/lib/stylesheetRetry.dom.test.ts

@@ -0,0 +1,105 @@
+// @vitest-environment jsdom
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { clear, events } from "./log";
+import { retryLostStylesheets } from "./stylesheetRetry";
+
+const LOADED = { cssRules: [{}] };
+// A lost stylesheet still has a `sheet`, and reading its rules throws (measured: Chromium, WebKit).
+const LOST = {
+  get cssRules(): never {
+    throw new DOMException("Cannot access rules", "SecurityError");
+  },
+};
+
+function link(href: string, loaded: boolean, sheet: unknown = loaded ? LOADED : LOST) {
+  const el = document.createElement("link");
+  el.rel = "stylesheet";
+  el.href = href;
+  el.setAttribute("crossorigin", "");
+  Object.defineProperty(el, "sheet", { value: sheet, configurable: true });
+  document.head.append(el);
+  return el;
+}
+
+const links = () => [...document.head.querySelectorAll<HTMLLinkElement>('link[rel="stylesheet"]')];
+const logged = () => events().map((e) => e.event);
+
+describe("retryLostStylesheets", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    clear();
+  });
+  afterEach(() => {
+    vi.useRealTimers();
+    document.head.innerHTML = "";
+  });
+
+  it("leaves a loaded stylesheet alone", () => {
+    const fine = link("/a.css", true);
+    retryLostStylesheets();
+    vi.runAllTimers();
+    expect(links()).toEqual([fine]);
+    expect(logged()).toEqual([]);
+  });
+
+  it("asks again for a lost one, in its place, keeping its attributes", () => {
+    const first = link("/a.css", true);
+    const lost = link("/b.css", false);
+    const last = link("/c.css", true);
+    retryLostStylesheets();
+    vi.advanceTimersByTime(0);
+    const [a, b, c] = links();
+    expect(a).toBe(first);
+    expect(b).not.toBe(lost);
+    expect(b!.getAttribute("href")).toBe("/b.css");
+    expect(b!.hasAttribute("crossorigin")).toBe(true);
+    expect(c).toBe(last);
+    expect(logged()).toEqual(["app.stylesheet_lost"]);
+  });
+
+  it.each([
+    ["no sheet at all", null],
+    ["an empty sheet", { cssRules: [] }],
+  ])("takes a link with %s for lost too", (_, sheet) => {
+    const lost = link("/a.css", false, sheet);
+    retryLostStylesheets();
+    vi.advanceTimersByTime(0);
+    expect(links()[0]).not.toBe(lost);
+  });
+
+  it("says so once the new request has loaded", () => {
+    link("/a.css", false);
+    retryLostStylesheets();
+    vi.advanceTimersByTime(0);
+    links()[0]!.dispatchEvent(new Event("load"));
+    vi.runAllTimers();
+    expect(logged()).toEqual(["app.stylesheet_lost", "app.stylesheet_recovered"]);
+  });
+
+  it("waits longer each time and then gives up, reporting it", () => {
+    link("/a.css", false);
+    retryLostStylesheets();
+    const fail = () => links()[0]!.dispatchEvent(new Event("error"));
+
+    vi.advanceTimersByTime(0);
+    const second = links()[0];
+    fail();
+    vi.advanceTimersByTime(999);
+    expect(links()[0]).toBe(second);
+    vi.advanceTimersByTime(1);
+    const third = links()[0];
+    expect(third).not.toBe(second);
+
+    fail();
+    vi.advanceTimersByTime(2999);
+    expect(links()[0]).toBe(third);
+    vi.advanceTimersByTime(1);
+    const fourth = links()[0];
+    expect(fourth).not.toBe(third);
+
+    fail();
+    vi.runAllTimers();
+    expect(links()[0]).toBe(fourth);
+    expect(logged()).toEqual(["app.stylesheet_lost", "app.stylesheet_gave_up"]);
+  });
+});

frontend/src/lib/stylesheetRetry.ts

@@ -0,0 +1,50 @@
+// A stylesheet can be lost on the way (a poor line, a loaded machine), and a browser never asks again
+// for one by itself: the page then stays unstyled until a reload, every control a bare browser button.
+// A module script waits for the document's stylesheets (measured: a sheet held 3 s held the module
+// too), so by the time this runs each link has either loaded or failed, and no event is left to wait
+// for. A lost one is asked for again in its own place, so the cascade's order stays as it was built.
+//
+// All of these sheets are this site's own: reading the rules of a loaded one is allowed, so a throw
+// says it failed (measured: Chromium, WebKit). Three tries, then the page stays as it is. A reload here
+// could loop, and the unstyled page still works.
+import { log } from "./log";
+
+const RETRY_AFTER_MS = [0, 1000, 3000];
+
+function holdsRules(link: HTMLLinkElement): boolean {
+  try {
+    return (link.sheet?.cssRules.length ?? 0) > 0;
+  } catch {
+    return false;
+  }
+}
+
+function askAgain(link: HTMLLinkElement, attempt: number): void {
+  const wait = RETRY_AFTER_MS[attempt];
+  if (wait === undefined) {
+    log.error("app.stylesheet_gave_up", "a stylesheet could not be loaded", { href: link.href });
+    return;
+  }
+  window.setTimeout(() => {
+    const next = link.cloneNode(false) as HTMLLinkElement;
+    next.addEventListener("load", () =>
+      log.info("app.stylesheet_recovered", "a lost stylesheet loaded on a new request", {
+        href: next.href,
+        attempt: attempt + 1,
+      }),
+    );
+    next.addEventListener("error", () => askAgain(next, attempt + 1));
+    link.replaceWith(next);
+  }, wait);
+}
+
+/** Run once, from a page's entry module. */
+export function retryLostStylesheets(): void {
+  for (const link of document.querySelectorAll<HTMLLinkElement>('link[rel="stylesheet"]')) {
+    if (holdsRules(link)) continue;
+    log.warn("app.stylesheet_lost", "a stylesheet was lost on the way; asking again", {
+      href: link.href,
+    });
+    askAgain(link, 0);
+  }
+}

frontend/src/main.tsx

@@ -17,12 +17,15 @@ import "flag-icons/css/flag-icons.min.css";
 import { lazyWithPreload } from "./lib/lazyWithPreload";
 import { installClientLog } from "./lib/logHooks";
 import { installStaleBuildReload } from "./lib/staleBuild";
+import { retryLostStylesheets } from "./lib/stylesheetRetry";
 import { appCopyOf } from "./i18n/appCopy";
 
 // First, before any render: a crash or a navigation during boot belongs in the log too.
 installClientLog(appCopyOf, typedUrlKeys());
 // A tab from before the last release reloads onto the new build instead of crashing on a lazy chunk.
 installStaleBuildReload();
+// A stylesheet lost on the way is asked for again rather than leaving the page unstyled.
+retryLostStylesheets();
 
 // Split by top-level route so each entry point is its own chunk: a legal page never downloads the
 // authenticated app bundle, and the app never carries it either. The legal pages live outside the

frontend/src/watchMain.tsx

@@ -5,6 +5,7 @@ import WatchPage from "./watch/WatchPage";
 import { T } from "./watch/watchStrings";
 import "./index.css";
 import { installStaleBuildReload } from "./lib/staleBuild";
+import { retryLostStylesheets } from "./lib/stylesheetRetry";
 
 // The entry of a public /watch/<token> link (watch.html). Whoever opens it got the link from a
 // friend: signed out, often on a phone, often on mobile data — so it imports the player page and
@@ -14,6 +15,8 @@ import { installStaleBuildReload } from "./lib/staleBuild";
 
 // A tab left open across a release reloads onto the new build instead of failing on a chunk.
 installStaleBuildReload();
+// A stylesheet lost on the way is asked for again rather than leaving the page unstyled.
+retryLostStylesheets();
 
 const crashCopy = () => ({ title: T.crashTitle, subtitle: T.crashHint, reload: T.reload });