Fail an E2E test on anything the browser logs, in all three engines

e2e-tests/console-guard.ts

@@ -0,0 +1,54 @@
+import { expect, test as base, type BrowserContext, type Page } from "@playwright/test";
+
+// The `test` every spec imports: it fails a test whose browser logged a console error or warning,
+// or threw, at any step — in whichever engine ran it. A zero-JS app has nothing to say in the
+// console, so anything there is a defect (a broken sub-resource, a rejected attribute, an engine
+// refusing a feature) that no assertion looks for.
+//
+// One module-level buffer is enough: a Playwright worker runs one test at a time, so the reset at
+// setup and the assertion at teardown bracket exactly the test in between.
+const problems: string[] = [];
+const allowed: RegExp[] = [];
+
+// The one message the stack itself provokes: the runner reaches `web`/`proxy` by container name over
+// plain http, and only a `localhost` origin is trustworthy without TLS — so Chromium drops the COOP
+// header the app sends and says so on every page. Over https, where a deployment serves, it applies.
+const EXPECTED = [/^console\.error: The Cross-Origin-Opener-Policy header has been ignored/];
+
+// Allow a message for the current test only, when the page under test provokes it on purpose.
+export function allowConsole(...patterns: RegExp[]): void {
+  allowed.push(...patterns);
+}
+
+function watch(page: Page): void {
+  page.on("console", (msg) => {
+    const type = msg.type();
+    if (type === "error" || type === "warning") problems.push(`console.${type}: ${msg.text()}`);
+  });
+  page.on("pageerror", (err) => problems.push(`pageerror: ${err.message}`));
+}
+
+// Every page of the context, however it is opened — `context.newPage()` fires this event too, so
+// watching the context is the whole job and a page must never be watched a second time on top.
+function watchContext(context: BrowserContext): BrowserContext {
+  context.on("page", watch);
+  return context;
+}
+
+// A spec that opens its own context — a page shared across a serial describe — goes through this.
+export function watchedPage(context: BrowserContext): Promise<Page> {
+  return watchContext(context).newPage();
+}
+
+export const test = base.extend<{ consoleGuard: void }>({
+  context: async ({ context }, use) => { await use(watchContext(context)); },
+  consoleGuard: [async ({}, use) => {
+    problems.length = 0;
+    allowed.length = 0;
+    await use();
+    const unexpected = problems.filter((p) => ![...EXPECTED, ...allowed].some((re) => re.test(p)));
+    expect(unexpected, "the browser logged nothing while this test ran").toEqual([]);
+  }, { auto: true }],
+});
+
+export { expect };