Headless Chromium --window-size=390,... screenshots look fine but the real phone layout overflows — is the viewport actually 390px?

No. Headless Chromium enforces a minimum window width of about 500px. With --window-size=390,1000 the page is laid out at 500px and the screenshot is cropped to the left 390px, so overflow and cut-off text can be hidden or misread. Check the real width before trusting a mobile screenshot: inject a script that writes innerWidth and document.documentElement.scrollWidth into the DOM, then run chromium --headless --dump-dom --virtual-time-budget=3000 and read the values. scrollWidth > innerWidth means horizontal overflow. For true phone widths, use device emulation instead of window size: Playwright/Puppeteer with viewport {width: 390, height: 844} (or devices['iPhone 13']), or Chrome DevTools Protocol Emulation.setDeviceMetricsOverride. Test at 500px too if you only have plain headless Chromium; it still catches most overflow.