Oh huh, that’s interesting, I just took the default value you get from the share dialog (under the ‘embed’ option) and added in the hideSimButtons parameter. Looks like a little bit of padding stuck around and nobody fixed it / just worked around it 
I’ll take a look at if it would be too much to fix it (as I think the kiosk looks like it has a very similar hack to what I’ll show below, which isn’t a big deal but have to make sure to fix everywhere).
But at the end of the day, it’s all just html/css that’s being applied, so it’s very easy to hack around it to fix those things:
<body style="overflow:hidden;">
<script src="script.js"></script>
<iframe style="position:absolute;top:0;left:-2rem;width:calc(100vw + 2rem);height:100vh;overflow:hidden" src="https://arcade.makecode.com/v1.11.37---run?id=S35510-75592-74609-07291&hideSimButtons=1&fullscreen=1" allowfullscreen="allowfullscreen" sandbox="allow-popups allow-forms allow-scripts allow-same-origin" frameborder="0"></iframe>
will make it show up like this:
(worth a quick note I added two more things – the &fullscreen=1 parameter to make sure it takes up all allocated space, and I pegged the version down to the currently released site with the v1.11.37 in the url – that way future updates - perhaps one where we fix the hideSimButtons / fullscreen embed to have no horizontal padding that puts it off center and requires css fixes outside the iframe - won’t get picked up automatically / your game won’t randomly break if we fix a bug but it changes something subtle for you.)