[Webkit-unassigned] [Bug 220215] New: macOS 11 Big Sur Regression: WebKit transform scaling of an image that is clipped appears choppy (non-fractional)

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=220215

            Bug ID: 220215
           Summary: macOS 11 Big Sur Regression: WebKit transform scaling
                    of an image that is clipped appears choppy
                    (non-fractional)
           Product: WebKit
           Version: Safari 14
          Hardware: Mac (Apple Silicon)
                OS: macOS 11
            Status: NEW
          Severity: Major
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: jonathan at tumult.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 416873

  --> https://bugs.webkit.org/attachment.cgi?id=416873&action=review

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 at 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/

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