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<!DOCTYPE html>
<html lang="en"><head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8"><meta charset="utf-8">
<title>Testcase for checkerboarding during horizontal scrolling</title>
<script type="application/javascript" src="apz_test_utils.js"></script>
<script type="application/javascript" src="apz_test_native_event_utils.js"></script>
<script src="/tests/SimpleTest/paint_listener.js"></script>
.scrollbox {
margin: 50px;
border: 2px solid black;
background: red;
width: 1120px;
height: 200px;
overflow: auto;
.scrolled {
width: 20000px;
height: 200px;
background: lime;
<div class="scrollbox"><div class="scrolled"></div></div>
<script type="application/javascript">
async function test() {
var scroller = document.querySelector(".scrollbox");
var utils = SpecialPowers.getDOMWindowUtils(window);
var scrollerId = utils.getViewId(scroller);
// This test contains a wide horizontal scroll box and scrolls it horizontally
// from right to left. The size of the box is chosen so that the displayport
// snapping logic in nsLayoutUtils.cpp would tries an horizontal alignment larger
// than the margins. In such a situation we want to make sure the displayport
// alignment is adjusted so we don't snap too far which would cause content to
// be missed on the right side.
// The scroll values here just need to be "thorough" enough to exercise the
// code at different alignments, so using a non-power-of-two or prime number
// for the increment seems like a good idea. The smaller the increment, the
// longer the test takes to run (because more iterations) so we don't want it
// too small either.
// The scroll box is rather wide so we only scroll a portion of it so that the
// test doesn't run for too long.
var maxX = scroller.scrollLeftMax / 6;
for (var x = maxX; x > 0; x -= 71) {
dump(`Scrolling scroller to ${x}\n`);
scroller.scrollTo(x, 0);
await promiseApzFlushedRepaints();
assertNotCheckerboarded(utils, scrollerId, `At x=${x}`);
.then(subtestDone, subtestFailed);