<!DOCTYPE html>
<html>
<meta charset="utf-8">
<link rel="help" href="https://drafts.csswg.org/css-scroll-anchoring-1/">
<head>
    <script src="/resources/testharness.js"></script>
    <script src="/resources/testharnessreport.js"></script>
    <style type="text/css">
        #scroller {
            overflow: scroll;
            width: 500px;
            height: 500px;
        }
        #anchor {
            position: relative;
            width: 100px;
            height: 100px;
            margin-top: 100px;
            margin-bottom: 1000px;
            background-color: blue;
        }
        #positioned {
            position: absolute;
            width: 10px;
            height: 10px;
            top: -200px;
            background-color: yellow;
        }
    </style>
</head>
<body>
    <div id="scroller">
        <div id="anchor">
            <div id="positioned">
            </div>
        </div>
    </div>
    <script type="text/javascript">
        test(() => {
            let scroller = document.querySelector('#scroller');
            let positioned = document.querySelector('#positioned');

            // Scroll down to select #anchor as an anchor node
            scroller.scrollTop = 20;

            // Move #positioned downwards, which will move the unclamped scrollable
            // overflow rect of #anchor downards as well
            positioned.style.top = '-180px';
            // To trigger the bug that this regression tests in Gecko, we need
            // to not take Gecko's relative positioning fast path. To do
            // this, change the 'left' of #positioned from 'auto' to '0px'.
            positioned.style.left = '0px';

            // The implementation should clamp the scrollable overflow rect
            // before the start-edge of the anchor node, and not apply an
            // adjustment
            assert_equals(scroller.scrollTop, 20);
        }, 'scrollable overflow before the start-edge of the anchor node should be clamped');
    </script>
</body>
</html>