test(acceptance): tell Firefox's interrupted image decode from a broken image (#713)

The batch 3 gate failed SEC-033 in Firefox on two console errors, 'Image
corrupt or truncated.' for samples-logo.png. The logo had been served
intact (200, then 304; the PNG's chunks and zlib stream check out). The
logout navigation cut off its decode, and Firefox reports that interrupted
decode as corruption. The diagnostics fixture now holds these reports until
the test ends, then has the same browser decode the image on the page's
origin. The report is recorded as an interrupted load only if that decode
succeeds. A really corrupt or truncated image, or one that cannot be checked,
still fails the test. Both outcomes were verified in Firefox.

tests/acceptance/support/fixtures.ts

@@ -135,8 +135,19 @@ export const test = base.extend<{ persona: Persona; user: SampleUser; backend: B
       if (target && nextRouteFetch(target)) reports.push({ text, at: performance.now(), push })
       else push()
     }
+    // Firefox logs "Image corrupt or truncated." for an intact image whose decode a navigation cut
+    // off (logging out while the logo decodes, for example). After the test, such a report is ignored
+    // only when the same browser, on the page's origin, decodes that image in full; an image that is
+    // really corrupt or truncated, or one that cannot be checked, still fails the test.
+    const truncatedImage = /^\[JavaScript Error: "Image corrupt or truncated\." \{file: "([^"]+)"/
+    const truncatedImages: { text: string; url: string }[] = []
     page.on('pageerror', (error) => report(error.message, () => diagnostics.pageErrors.push(error.message)))
-    page.on('console', (message) => { if (message.type() === 'error') report(message.text(), () => diagnostics.consoleErrors.push(message.text())) })
+    page.on('console', (message) => {
+      if (message.type() !== 'error') return
+      const image = truncatedImage.exec(message.text())?.[1]
+      if (image) truncatedImages.push({ text: message.text(), url: image })
+      else report(message.text(), () => diagnostics.consoleErrors.push(message.text()))
+    })
     page.on('requestfailed', (request) => {
       const failure = request.failure()?.errorText ?? ''
       // Navigation-cancelled background reads are not application failures.
@@ -158,6 +169,16 @@ export const test = base.extend<{ persona: Persona; user: SampleUser; backend: B
     // One round trip delivers violation reports still queued in the page.
     if (!page.isClosed()) await page.evaluate(() => 0).catch(() => undefined)
     classifyAccessControlReports()
+    for (const { text, url } of truncatedImages) {
+      const decodes = !page.isClosed() && new URL(url).origin === new URL(page.url()).origin && await page.evaluate((src) => new Promise<boolean>((resolve) => {
+        const image = new Image()
+        image.onload = () => { void image.decode().then(() => resolve(image.naturalWidth > 0), () => resolve(false)) }
+        image.onerror = () => resolve(false)
+        image.src = src
+      }), url).catch(() => false)
+      if (decodes) diagnostics.interruptedFetches.push(text)
+      else diagnostics.consoleErrors.push(text)
+    }
     const unexpected = [
       ...diagnostics.pageErrors.map((text) => `pageerror: ${text}`),
       ...diagnostics.consoleErrors.map((text) => `console: ${text}`),