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<meta charset="utf-8">
<meta name="viewport" content="width=device-width, minimum-scale=1.0">
<title>Tests scroll position is properly synchronized when visual position is temporarily clamped on the main thread</title>
<script src="apz_test_utils.js"></script>
<script src="/tests/SimpleTest/paint_listener.js"></script>
.hoverthingy, button {
width: 100%;
height: 200px;
text-align: center;
border: solid 1px black;
background-color: white;
.hoverthingy:hover {
background-color: lightgray;
<div id="filler" style="height: 5000px">This test runs automatically in automation. To run manually, follow the steps: 1. scroll all the way down</div>
<div class="hoverthingy">3. move the mouse. this div should have a hover effect exactly when the mouse is on top of it</div>
<button onclick="clampRestore()">2. click this button</div>
/* eslint-disable no-unused-vars */
function clampRestore() {
// Shorten doc to clamp scroll position
let filler = document.getElementById('filler'); = '4800px';
// Force scroll position update
let scrollPos = document.scrollingElement.scrollTop;
// Restore height = '5000px';
function getAsyncScrollOffset() {
let apzcTree = getLastApzcTree();
let rcd = findRcdNode(apzcTree);
if (rcd == null) {
return {x: -1, y: -1};
return parsePoint(rcd.asyncScrollOffset);
async function test() {
document.scrollingElement.scrollTop = document.scrollingElement.scrollTopMax;
await promiseApzFlushedRepaints();
await promiseApzFlushedRepaints();
let apzScrollOffset = getAsyncScrollOffset();
dump(`Got apzScrollOffset ${JSON.stringify(apzScrollOffset)}\n`);
// The bug this test is exercising resulted in a situation where the
// main-thread scroll offset and the APZ scroll offset remained out of sync
// while in the steady state. This resulted mouse hover effects and clicks
// being offset from where the user visually saw the content/mouse. We
// check to make sure the scroll offset is in sync to ensure the bug is fixed.
is(apzScrollOffset.y, document.scrollingElement.scrollTop,
"RCD y-scroll should match between APZ and main thread");
.then(subtestDone, subtestFailed);