Embedding Google Apps Script in an iFrame

I was having this issue on the doPost response form, only in dev mode, and corrected it by changing target="_self" to target="_top" on the initial doGet form.

   <form method="POST" id="ss-form" target="_top" action="<?!=SETTINGS.PUBLISHED_URL?>">
   

Google had just recently enabled this feature. It has been under a 'feature request' status for quite a long time. Link here

You can now explicitly define X-Frame-Options.

To allow embedding under another domain, the option should be HtmlService.XFrameOptionsMode.ALLOWALL

Google documentation on the subject:

https://developers.google.com/apps-script/reference/html/html-output#setXFrameOptionsMode(XFrameOptionsMode)

Example:

function doGet() {
    return HtmlService.createTemplateFromFile('form.html')
        .evaluate() // evaluate MUST come before setting the Sandbox mode
        .setXFrameOptionsMode(HtmlService.XFrameOptionsMode.ALLOWALL);
}

Hope this helps!