Thanks for the repro! I investigated it a bit and while I’m not 100% sure, it seems to be some CSS weirdness going on 
It looks like that because the iframe has height: 100%, it is trying to occupy the full page, I’m just not sure why this only happens when it is focused on. However, by messing around I found that if you set display:flex; flex-direction: column; on the body style, it stops trying to do that. My solution for the weirdness that is CSS is usually flexbox 
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