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[GForm Limitation]
=Cannot make form background transparent.
=Cannot pass a hidden field
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When you put a Google Form in an iframe, you will get errors like:
GET filesystem:https://docs.google.com/persistent/docs/fonts/KFOmCnqEu92Fr1Me4A.woff2 net::ERR_FILE_NOT_FOUND
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You can not remove this even if you put the parent page in HTTPS.
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[Solution]
I think there is no solution for this because "filesystem:" is obsolete and not standard.
Firefox doesn't support the "filesystem:" URL scheme.
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Another solution is run console.clear() on load of iframe
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<script>
let idx=0;
const el_iframe=document.querySelector(".gform");
el_iframe.addEventListener("load",function(){
	console.clear();
	console.log("["+idx+"] loaded: error messages were cleared on el_iframe.addEventlistener('load')");
	idx++;
	// this does not work because of the same origin policy
	console.log(el_iframe.contentWindow.location.href);
});
</script>
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el_iframe.contentWindow.location.href will cause the following error message in Chrome
==>
test.php:75 Uncaught DOMException: Failed to read a named property 'href' from 'Location': Blocked a frame with origin "http://php80ub20.solar.lan" from accessing a cross-origin frame.
    at HTMLIFrameElement.<anonymous> (http://php80ub20.solar.lan/phpkobo/research/js-experimental/kenshow/jungle-quiz/CGForm/test.php:75:47)
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el_iframe.contentWindow.location.href will cause the following error message in FireFox
==>
Uncaught DOMException: Permission denied to get property "href" on cross-origin object
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How to detect if the user submit the form?
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It's not possible to find the change of URL of the iframe because of the same origin policy.
`el_iframe.contentWindow.location` will not work.
You can count how many times the load event has occured. If it's more than one,
then the form must've been submitted.
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