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<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Re-snapping to the last snapped element on APZ</title>
<script src="apz_test_utils.js"></script>
<script src="apz_test_native_event_utils.js"></script>
<script src="/tests/SimpleTest/paint_listener.js"></script>
body {
margin: 0;
div {
position: absolute;
#scroller {
width: 100%;
height: 500px;
overflow-y: scroll;
scroll-snap-type: y proximity; /* to escape from the last snap point */
.child {
width: 100%;
height: 100px;
background-color: blue;
scroll-snap-align: start;
<div id="scroller">
<div class="child" style="top: 0px;"></div>
<div class="child" style="top: 200px;"></div>
<div id="spacer" style="width: 100%; height: 2000px;"></div>
<script type="application/javascript">
async function test() {
const proximityThreshold =
let transformEndPromise = promiseTransformEnd();
await promiseMoveMouseAndScrollWheelOver(scroller, 100, 100);
await transformEndPromise;
await promiseApzFlushedRepaints();
is(scroller.scrollTop, 200, "snap to 200px");
document.querySelectorAll(".child")[1] = "400px";
is(scroller.scrollTop, 400, "re-snap to 400px");
// Make sure the new snap position has been reflected in APZ side.
await promiseApzFlushedRepaints();
// Now trigger another wheel scroll to escape from the last snap point.
transformEndPromise = promiseTransformEnd();
// With a small `layout.css.scroll-behavior.spring-constant` preference
// value (it's set in test_group_scroll_snap.html), an async scroll
// operation, e.g. a wheel scroll operation, fires multiple scroll events
// so that here we wait the first scroll event here by specifying
// `waitForScroll=true`, there should be other scroll events after the
// first one.
await promiseMoveMouseAndScrollWheelOver(scroller, 100, 100,
true /* waitForScroll */, proximityThreshold + 1000);
// Expand the scrollable region during the wheel scroll. = "2200px";
await transformEndPromise;
await promiseApzFlushedRepaints();
isnot(scroller.scrollTop, 400, "Do not re-snap to the original 400px point");
.then(subtestDone, subtestFailed);