Actually issue is that Edge iframe scrolling defaults to 'auto' like IE does, and unlike other browsers, even though actually it's not even part of HTML5 anymore so invalid markup to turn it off. Our JS sees scrolling is there so doesn't try and resize the preview frame to fit it's content height. Fix is to put in scrolling="no" based on improved UA sniffing and also to resize height if it is zero even if scrolling is on..