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<!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>
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