Rendering Chromium To Png

rendering-chromium-to-png

Type-checking and tests verify code correctness, not feature correctness — a UI can be green on tsc/vitest and render broken (empty section, stuck spinner, wrong colors, a throw that aborts the render partway). This skill gives you eyes: render to a PNG, then Read the PNG so the actual pixels enter context. It's the HOW behind the fleet's "verify rendered output before commit" rule.

Page mode — render any URL or local file

For a locally served app, run its existing development command through the package.json script that launches it through portless, then pass the generated HTTPS .localhost URL to this script. Do not invent a fixed port or a second server command. Use file: only for a genuinely static page.

node .claude/skills/fleet/rendering-chromium-to-png/screenshot.mts <url|file> \
  [--out p.png] [--width 580] [--height 0=full] [--theme dark|light] [--wait 2500] [--full]

Then Read the --out PNG. Defaults: 580px wide, full-page, dark theme, 2.5s settle.

Extension mode — load a real unpacked MV3 extension

node .claude/skills/fleet/rendering-chromium-to-png/screenshot.mts \
  --extension <unpacked-dir> [--page popup.html] [--out p.png] [--width 580] [--theme dark|light]

This loads the extension with its REAL powers — background service worker, content scripts, chrome.* APIs — via launchPersistentContext + channel: 'chromium' (the documented way to run extensions in headless Chromium; plain headless silently ignores --load-extension). It resolves the extension id from the background service worker, navigates to chrome-extension://<id>/<page> (popup by default), and screenshots it. Use this to see an extension popup as it actually renders in-browser, not a static file:// approximation.

How you actually "see" it

1. Run the script → it writes a PNG.
2. Read that PNG path. The harness decodes the image; the rendered pixels go into your context. You observe the UI the same as a human looking at a screenshot.

Caveats (state these honestly in your summary)

• Static snapshot, not interactive. You can't hover/click/scroll in one shot. For a state behind a click, drive it — a small playwright script that clicks then screenshots — or capture different states by timing the --wait. Each state = its own screenshot.
• Mock vs live data. If the page needs a backend that isn't running, you're seeing empty/placeholder states — say so. (A built-in ?preview mock, if the app has one, is still mock data; the layout/colors/bugs are real, the content isn't.)
• MV3 service workers suspend after ~30s idle and restart on demand — long-lived evaluate() may throw "Service worker restarted"; keep interactions short.
• No browser available (headless CI without chromium): say so explicitly rather than claiming you verified — run node_modules/.bin/playwright install chromium first.

Browser dependency

playwright-core (fleet catalog devDep) drives a headless Chromium. If the binary is missing the script says so — install it with node_modules/.bin/playwright install chromium.