Just in case anyone else struggles with this, here’s how you remove the scrollbar and border from an iframe wrapping an anvil app :
<iframe
style="border:0;"
scrolling="no"
data-anvil-embed
width="100%"
src="https://xxxxx.anvil.app">
</iframe>
EVERY SINGLE RESULT on the first 3 pages of google either stretches back to 2012 where they recommend “seamless”, or they tell you “don’t use seamless!” but offer no alternative.
Two hours I won’t get back …