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<!DOCTYPE html>
<html>
<meta name="viewport" content="width=device-width">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<link rel="author" title="Mozilla" href="https://mozilla.org">
<link rel="help" href="https://drafts.csswg.org/css-scroll-anchoring/">
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1856088">
<div style="height: 500vh;"></div>
<div id="anchor" style="height: 10px; background-color: blue;"></div>
<div style="height: 200vh;"></div>
<script>
promise_test(async t => {
assert_equals(window.scrollY, 0);
let anchorRect = anchor.getBoundingClientRect();
// Scroll to the anchor node.
window.scrollBy(0, anchorRect.y);
assert_equals(window.scrollY, anchorRect.y);
await new Promise(resolve => t.step_timeout(resolve, 0));
// Shrink the first element so that scroll anchoring happens.
document.querySelectorAll("div")[0].style.height = "200vh";
// Flush the change.
document.querySelectorAll("div")[0].getBoundingClientRect();
await new Promise(resolve => window.addEventListener("scroll", resolve));
// Revert the height change in a scroll event handler.
document.querySelectorAll("div")[0].style.height = "500vh";
await new Promise(resolve => requestAnimationFrame(resolve));
await new Promise(resolve => requestAnimationFrame(resolve));
assert_equals(window.scrollY, anchorRect.y);
}, "Scroll anchoring properly works after scrollable range shrinkage");
</script>
</html>
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