I'm glad it's working for you in Microsoft Edge!
Many of us here at RStudio use rstudio.cloud with Google Chrome just fine and I know Microsoft Edge is using the same rendering engine as it is now Chromium based as well. So I'm sadly not entirely sure why it's acting weird for you. If I had to guess, there is probably a browser plugin you have running in Google Chrome that is interfering with the CSS.
If you know how to use Inspect Element in Chrome you can check the CSS rule to see if they show up as disabled. When I disable the width and height CSS rules for #contentIFrame I get the same result as your first screenshot.