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Headless Chrome hides scrollbars, so no test saw the phone preview freeze: test the host's frame with real ones

Rule

If a page may behave differently where scrollbars take room, test it with Playwright's --hide-scrollbars removed, in a frame at the size the host shows it, in a desktop-sized window.

Snippet
import { chromium } from "playwright-core";
// before: chromium.launch({ channel: "chrome", headless: true }), which adds --hide-scrollbars
const browser = await chromium.launch({ channel: "chrome", headless: true, ignoreDefaultArgs: ["--hide-scrollbars"] });
try {
  const page = await browser.newPage({ viewport: { width: 1280, height: 1000 } });
  // The host's preview: the built page in a sandboxed frame at the phone width it offers.
  await page.setContent(`<iframe id="f" sandbox="allow-scripts" src="http://localhost:6702/preview.html"
    style="width:390px;height:765px;border:0"></iframe>`);
  const frame = await (await page.waitForSelector("#f")).contentFrame();
  await frame.waitForSelector('.fl-body[data-gl="ready"]');
  const bar = await frame.evaluate(() => innerWidth - document.documentElement.clientWidth);
  console.log("scrollbar", bar); // 15 here on Windows; always 0 with the default launch
} finally {
  await browser.close();
}
Check

the logged scrollbar width is above 0 (15 px here). Then click a shoe: its panel shows within 2 s. Before the fix it never showed in 3 s, as on the live page.

Applies to

other: headless Chromium from Playwright (seen with playwright-core 1.64)

I assumed

a headless page at a phone's width shows what the site's phone-width preview shows.

What happened

every check had passed, and the live page had passed a headless click minutes earlier. In a frame built like the site's, with real scrollbars, the same build showed the reported screen exactly: the camera arrived and the sheet stayed empty.

The fix

scripts/qa/site-frame.mjs: a desktop window, the built preview in a sandboxed frame at 390 and 768 px, the preview's own policy, and real scrollbars.

Evidence

commit 6fc7667.

Learned in the articleA camera that walks up to a shoe: springs, buying inside a 3D room, and a wall placed by CSS alone

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