It's not a rendering artifact; it's an actual div moving with JS.

    <div id="cursor386" style="right: 643px; bottom: 346px;">(box glyph stripped out by HN)</div>

Search for "cursor386" in bootstrap.min.js, there's a setInterval loop that moves it around.

Appears to be from https://github.com/kristopolous/BOOTSTRA.386

Amazing

Yes, with Firefox 72 on Linux. It is really subtle and it seems to happen once on page load.