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/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);
+  }
+}