If a same domain iframe is created using `srcdoc` with `sandbox=allow-same-origin` and an event listener is created from the parent page e.g.
const img = iframe.contentDocument.querySelector('img');
The event will be blocked in Safari with the error `Blocked script execution in 'about:srcdoc' because the document's frame is sandboxed and the 'allow-scripts' permission is not set.` In other modern browsers the event will be received by the parent page. Adding 'allow-scripts' will allow the parent to receive the event.
We believe according to the spec https://html.spec.whatwg.org/multipage/webappapis.html#sandboxScriptBlocked these events should not be blocked as they execute in the parent window and not the sandboxed iframe window.
You can find a demo at: https://sandboxed-listener.glitch.me/
Thanks for filing, I can reproduce this on Safari 12.1.2 as well as Safari Beta version 14.0.1 using the linked test case.
*** Bug 221283 has been marked as a duplicate of this bug. ***
This bug also exists in Safari Version 15.5.
This bug has a negative affect on web security because Safari's behavior means it's harder for developers to utilize sandboxed iframes to block scripts from within the iframe, if the external page still needs any scripting control.