Created attachment 416873 [details]
Xcode reproduction project

This is a Big Sur regression.

Scaling an element with an image that is clipped (say by a parent with overflow:hidden) will not render properly on fractional numbers.  The result is that when animating this appears very choppy.

This affects mac applications using the WebKit framework, but not WKWebView or Safari.

I'm attaching a sample project and video demonstrating how bad this looks. The HTML code is:

<style>
#container {
	width: 540px;
	height: 540px;
	overflow: hidden;
}
#child {
	width: 540px;
	height: 540px;
	background-image: url('daiyu01@2x.jpg');
	background-repeat: no-repeat no-repeat;
}
</style>

<div id = "container">
	<div id="child"></div>
</div>
    
<script>
var scale = 1.2;
function heartbeat() {
	scale -= .0004;
	document.getElementById("child").style.transform = "scale(" + scale + ")";
}
window.setInterval(heartbeat, 17);
</script>

This was originally reported by a user of the HTML5 animation tool Tumult Hype: https://tumult.com/hype/