This is a screenshot of https://rstudio.github.io/bslib/articles/value-boxes.html:

I'm on Chrome on Ubuntu, on a 4K screen.
It seems to be the overflow: auto rules we added to .html-fill-container and .html-fill-container > .html-fill-item that's causing this. Adding an inline style with overflow: hidden to the .value-box-showcase element fixes it (I didn't try it with a legit CSS rule).