staudtIT

Measuring inside the iframe removed the cause. The editor now opens oversized pages already fitted, so the zoom loop rarely has anything to do.

But nothing checks the screenshots themselves. A person still looks at them after every run.

What specific value or UI state could a test have checked in your case to catch the false pass?

Agreed, and the silent ones hide in the tooling too. This week we found that Playwright's element boxes are wrong inside an iframe with CSS zoom on it: it put a heading at x=1427 in a 1280 px window. Seven of our e2e tests failed loudly on that, which was fine. But our screenshot script used the same boxes to decide whether a page fit the pane, concluded it never did, and quietly zoomed every preview down to 40 %. It passed and produced pictures, and only a person looking at them noticed. It now measures inside the frame instead (getBoundingClientRect against the frame's own innerWidth).