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Test Info:

<title>Test that scaled elements with scrolled contents don't repaint unnecessarily when we scroll inside them</title>
<script src="/tests/SimpleTest/SimpleTest.js"></script>
<script src="/tests/SimpleTest/paint_listener.js"></script>
<link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
<body onload="setPrefAndStartTest()">
<div id="t" style="transform: scale(1.2, 1.2); transform-origin:top left; width:200px; height:500px; background:yellow; overflow:auto">
<div style="height:40px;">Hello</div>
<div id="e" style="height:30px; background:lime">Kitty</div>
<div style="height:800px; background:yellow">Kitty</div>
<pre id="test">
<script type="application/javascript">
var t = document.getElementById("t");
var e = document.getElementById("e");
var utils = SpecialPowers.getDOMWindowUtils(window);
function startTest() {
// Do a couple of scrolls to ensure we've triggered activity heuristics.
waitForAllPaintsFlushed(function () {
t.scrollTop = 5;
waitForAllPaintsFlushed(function () {
t.scrollTop = 10;
waitForAllPaintsFlushed(function () {
// Clear paint state now and scroll again.
t.scrollTop = 15;
waitForAllPaintsFlushed(function () {
var painted = utils.checkAndClearPaintedState(e);
is(painted, false, "Fully-visible scrolled element should not have been painted");
function setPrefAndStartTest() {
{"set": [["layers.single-tile.enabled", false]]},
// Need a timeout here to allow paint unsuppression before we start the test
function() {
setTimeout(startTest, 0);