Downloading draft of email using new email designer

​@bjohnson-5, I understand what you're trying to achieve. I'd still recommend using Send Proof to deliver the email directly to stakeholders' inboxes for approval. That’s what we follow at our organization. 

However, if you prefer sharing screenshots, one workaround is to send a test email to yourself and use GoFullPage to capture the entire email from your browser. hope this helps. 

You need to pop up the preview in its own window.

Open Dev Tools and run this in Console:

{
    const appMain = document.querySelector("iframe[id^='exc-app-sandbox-marketoEngage']");
    const nestedApp = appMain.contentDocument.querySelector("iframe[id='nested-app']");
    const previewPane = nestedApp.contentDocument.querySelector("iframe[srcdoc]");

    const previewHTML = previewPane.srcdoc;
    const blob = new Blob([previewHTML], { type: 'text/html' });
    const blobURL = URL.createObjectURL(blob);
    nestedApp.contentWindow.open(blobURL, '_blank');
}

I’ve tested this with both the Chrome GoFullPage extension you mention & with Chrome’s builtin “fullsize screenshot” feature.