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<!DOCTYPE html>
<head>
<title>Test that scroll anchoring is suppressed when scroll offset is zero.</title>
<link rel="author" title="Nick Burris" href="mailto:nburris@chromium.org">
<link rel="help" href="https://drafts.csswg.org/css-scroll-anchoring/">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
</head>
<style>
#header {
height: 100px;
border: 1px solid black;
overflow-anchor: none;
}
#content {
height: 200vh;
}
</style>
<div id="header"></div>
<div id="content">abc</div>
<script>
window.addEventListener("scroll", function() {
if (document.scrollingElement.scrollTop > 0) {
// On the first scroll event, shrink the header. Scroll anchoring anchors to
// content, but the header shrinks by more than the scroll offset so the
// resulting scroll position is zero.
step_timeout(function() {
document.querySelector("#header").style.height = "50px";
}, 0);
} else {
// On the second scroll event, grow the header. Since the scroll offset is
// zero, scroll anchoring should be suppressed. Otherwise, scroll anchoring
// would anchor to content and the resulting scroll position would be 50px.
step_timeout(function() {
document.querySelector("#header").style.height = "100px";
}, 0);
}
});
async_test(function(t) {
// Scroll down a bit to trigger the scroll event listener.
window.scrollTo(0, 10);
window.requestAnimationFrame(function() {
window.requestAnimationFrame(function() {
window.requestAnimationFrame(t.step_func_done(() => {
assert_equals(document.scrollingElement.scrollTop, 0);
}));
});
});
}, "Scroll anchoring suppressed when scroll offset is zero.");
</script>
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