It was a common tool for "clickjacking" experiments, where a refresh could reset the state of a transparent overlay. Why was it patched?

The standard XFO (X-Frame-Options) or CSP headers are now being strictly enforced, even during a forced refresh.

The browser may simply stop the frame from loading if it detects a ViewerFrame state change that violates security protocol. How to Move Forward

If you need to communicate between a parent and a child frame, use the window.postMessage API. It is the secure, modern standard.

If you were using this method for legitimate testing or niche web app functionality, you’ll likely see one of the following errors: