Headless Chrome --window-size=375 screenshot shows the desktop layout, cut off at 375 px

AO checks every page at phone width before it ships. On 2026-10-10 an entry page, captured with msedge --headless=new --window-size=375,3400 --screenshot, came out with every line cut off on the right, as if the page overflowed. It did not: the page that was already live, captured the same way, was cut off the same way. The browser had laid the page out wider than the image it saved.

What we measured, on one Windows 11 PC, 2026-10-10:

• Edge 155, --headless=new: a window of 375, or anything up to 520, gave a page 496 px wide. 600 gave 574.
• Chrome 154, --headless=new: 375 gave 504. 800 gave 782, and a height of 300 gave 204.
• chrome-headless-shell 149, the old headless mode that Playwright uses: exactly the size asked, every time.

What it does to a phone-width check. A page with a rule for max-width: 480px was captured at 375x812. Chrome and Edge drew the wide layout, and the shell drew the phone layout. So a screenshot taken this way at 375 px never shows the phone layout of a page whose breakpoint is below about 500 px. It shows the wide layout with its right side cut off, which looks like a page that overflows. Both are wrong answers to "does it work on a phone".

Why, as far as we can tell. New headless is the real browser with a window that is never shown, and on Windows that window keeps the rules of a real one: a minimum width, and a frame taken out of the size you ask for (18 to 26 px of width and 93 to 96 px of height here). That is our reading, not proven. The 2023 bug fits it: there, Windows' limit on a new window's size made new headless ignore a --window-size that was too large.

What to do.

• Use chrome-headless-shell for screenshots. Chrome's docs point to the Chrome for Testing downloads for it, Puppeteer runs it with headless: 'shell', and Playwright already runs it when headless.
• Or set the page's size, not the window's: that is what Playwright's and Puppeteer's viewport options are for. AO has not measured them in new headless.
• Or put the page in an iframe of the width you want, inside a window wider than about 520 px: it was laid out at 375.
• Either way, print innerWidth in the page you capture. If it is not the width you asked for, the screenshot is not a phone.

How to check your own machine (Git Bash; use Chrome's path instead of Edge's for Chrome):

echo '<script>document.write(innerWidth+" x "+innerHeight)</script>' > probe.html
"/c/Program Files (x86)/Microsoft/Edge/Application/msedge.exe" --headless=new --window-size=375,812 --dump-dom "file:///$(pwd -W)/probe.html"

Not measured. Linux and macOS, display scaling other than 100% (these runs reported a device pixel ratio of 1), Puppeteer and Playwright's viewport options in new headless, and other Chrome and Edge versions.