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<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Page scroll snaps a snap point in the same page rather than the one in the next page</title>
<script src="/tests/SimpleTest/EventUtils.js"></script>
<script type="application/javascript" src="apz_test_native_event_utils.js"></script>
<script type="application/javascript" src="apz_test_utils.js"></script>
<script src="/tests/SimpleTest/paint_listener.js"></script>
div {
position: absolute;
margin: 0px;
body {
margin: 0px;
html {
overflow-y: scroll;
scroll-snap-type: y mandatory;
.snap {
scroll-snap-align: start;
background: green;
<div class="snap" style="top: 0vh; width: 100%; height: 20px;">1</div>
<div class="snap" style="top: 50vh; width: 100%; height: 20px;">2</div>
<div class="snap" style="top: 102vh; width: 100%; height: 20px;">3</div>
<div class="snap" style="top: 300vh; width: 100%; height: 20px;">4</div>
<div class="snap" style="top: 400vh; width: 100%; height: 20px;">5</div>
<script type="application/javascript">
async function test() {
const expectedPosition =
const keyPromise = promiseOneEvent(window, "keydown", null);
await keyPromise;
let startedScroll = false;
let sameScrollPosition = false;
let prevScrollPos = window.scrollY;
while (true) {
// Flush APZ repaints to ensure that scroll offset changes from
// a compositor sample reach the content process.
await promiseApzFlushedRepaints();
let scrollPos = window.scrollY;
if (scrollPos == prevScrollPos) {
if (startedScroll) {
// If we had the same scroll position in two frames consecutively,
// we consider scroll has finished.
if (sameScrollPosition) {
sameScrollPosition = true;
} else {
sameScrollPosition = false;
if (!startedScroll && scrollPos > 0) {
startedScroll = true;
prevScrollPos = scrollPos;
// Use a fuzzy comparison with epsilon=1, because expectedPosition
// can be fractional (e.g. if viewport height is an odd number of pixels,
// then 50vh will be fractional), and ClampAndAlignWithPixels can round
// the final scroll offset requested by APZ at the end of an animation,
// to an integer value. We plan to remove ClampAndAlignWithPixels in
isfuzzy(window.scrollY, expectedPosition, 1,
"Snaps to the second snap point rather than the third snap point " +
"which was initially out of the scrollport even if it's closer to " +
"the top of the next page than the second snap point");
.then(subtestDone, subtestFailed);