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<title>Scrolling a scrollinfo layer and making sure it doesn't checkerboard</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>
<meta name="viewport" content="width=device-width"/>
#withfilter {
filter: url(#menushadow);
#scroller {
width: 300px;
height: 1038px;
overflow: scroll;
.spacer {
height: 1878px;
background-image: linear-gradient(red, blue);
<div id="withfilter">
<div id="scroller">
<div class="spacer"></div>
<!-- the SVG below copied directly from the Gecko Profiler code that
demonstrated the original bug. It basically generates a bit of a "drop
shadow" effect on the div it's applied to. Original SVG can be found at
<filter id="menushadow" color-interpolation-filters="sRGB" x="-10" y="-10" width="30" height="30">
<feComponentTransfer in="SourceAlpha">
<feFuncA type="linear" slope="0.3"/>
<feGaussianBlur stdDeviation="5"/>
<feOffset dy="10" result="shadow"/>
<feComponentTransfer in="SourceAlpha">
<feFuncA type="linear" slope="0.1"/>
<feMorphology operator="dilate" radius="0.5" result="rim"/>
<feMerge><feMergeNode in="shadow"/><feMergeNode in="rim"/></feMerge>
<feComposite operator="arithmetic" in2="SourceAlpha" k2="1" k3="-0.1"/>
<feMerge><feMergeNode/><feMergeNode in="SourceGraphic"/></feMerge>
<script type="application/javascript">
async function test() {
var scroller = document.querySelector("#scroller");
var utils = SpecialPowers.getDOMWindowUtils(window);
var scrollerId = utils.getViewId(scroller);
// Scroll to the bottom of the page, so that the bottom of #scroller is
// visible; that's where the checkerboarding happens.
document.scrollingElement.scrollTop = document.scrollingElement.scrollTopMax;
// After the first call to promiseApzFlushedRepaints, the scroller will have
// zero displayport margins (because it's inside an SVG filter, and so takes
// the "scroll info layer" codepath in APZ's CalculatePendingDisplayPort
// function. The main-thread then computes a displayport using those zero
// margins and alignment heuristics. If those heuristics are buggy, then the
// scroller may end up checkerboarding. That's what we check for on each
// scroll increment.
// 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.
for (var y = 3; y <= scroller.scrollTopMax; y += 17) {
dump(`Scrolling scroller to ${y}\n`);
scroller.scrollTo(0, y);
await promiseApzFlushedRepaints();
assertNotCheckerboarded(utils, scrollerId, `At y=${y}`);
.then(subtestDone, subtestFailed);