test(jassub): [PKG-JASSUB-09] verify composited subtitle screenshots

test/browser/jassub-display.js

@@ -0,0 +1,34 @@
+import { PNG } from 'pngjs'
+
+// Read the browser's composited PNG in Node, without copying the transferred canvas.
+export async function captureJassubDisplay(page, phase) {
+  const geometry = await page.evaluate(() => window.jassubGeometry())
+  const started = Date.now()
+  let png
+  try {
+    png = await page.screenshot({ clip: geometry.clip })
+  }
+  catch (error) {
+    await page.evaluate((failure) => {
+      (window.jassubNative.displayFailures ||= []).push(failure)
+    }, { message: error.message, elapsed: Date.now() - started, geometry })
+    throw error
+  }
+  const { data, width, height } = PNG.sync.read(png)
+  let visible = 0
+  let signature = 0
+  for (let i = 0; i < data.length; i += 4) {
+    // The screenshot diagnostic ASS uses green glyphs; normal fixtures stay white.
+    if (data[i] < 70 && data[i + 1] > 180 && data[i + 2] < 70) {
+      visible++
+      signature = (signature + i) >>> 0
+    }
+  }
+  const state = { ...geometry, phase, visible, signature, screenshotWidth: width, screenshotHeight: height, screenshotElapsed: Date.now() - started, observation: 'composited-png' }
+  await page.evaluate((state) => {
+    window.jassubNative.lastPixel = state
+    if (state.phase)
+      window.jassubNative.snapshots.push(state)
+  }, state)
+  return { state, png }
+}