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<title>Clicking on the scrollbar track in quick succession should scroll the right amount</title>
<script type="application/javascript" src="apz_test_native_event_utils.js"></script>
<script type="application/javascript" src="apz_test_utils.js"></script>
<script src="/tests/SimpleTest/paint_listener.js"></script>
<script src="/tests/SimpleTest/EventUtils.js"></script>
<script type="application/javascript">
// A helper to synthesize a native mouse click on a scrollbar track,
// and wait long enough such that subsequent ticking of the refresh
// driver will progress any resulting scroll animation.
// In particular, just `await promiseNativeMouseEventWithApz(...)`
// is not enough, it waits for the synthesization messages to arrive
// in the parent process, but the native events may still be in the
// OS event queue. Instead, we need to wait for a synthesized event
// to arrive at content. While we're synthesizing a "click", if the
// target is a scrollbar the window only gets the "mousedown" and
// "mouseup", not a "click". Waiting for the "mousedown" is not enough
// (the "mouseup" can still be stuck in the event queue), so we wait
// for "mouseup".
async function promiseNativeMouseClickOnScrollbarTrack(anchor, xOffset, yOffset) {
await promiseNativeMouseEventWithAPZAndWaitForEvent({
type: "click",
target: anchor,
offsetX: xOffset,
offsetY: yOffset,
eventTypeToWait: "mouseup"
async function test() {
var scroller = document.documentElement;
var verticalScrollbarWidth = window.innerWidth - scroller.clientWidth;
if (verticalScrollbarWidth == 0) {
ok(true, "Scrollbar width is zero on this platform, test is useless here");
// The anchor is the fixed-pos div that we use to calculate coordinates to
// click on the scrollbar. That way we don't have to recompute coordinates
// as the page scrolls. The anchor is at the bottom-right corner of the
// content area.
var anchor = document.getElementById('anchor');
var xoffset = (verticalScrollbarWidth / 2);
// Get a y-coord near the bottom of the vertical scrollbar track. Assume the
// vertical thumb is near the top of the scrollback track (since scroll
// position starts off at zero) and won't get in the way. Also assume the
// down arrow button, if there is one, is square.
var yoffset = 0 - verticalScrollbarWidth - 5;
// Take control of the refresh driver
let utils = SpecialPowers.getDOMWindowUtils(window);
// Click at the bottom of the scrollbar track to trigger a page-down kind of
// scroll. This should use "desktop zooming" scrollbar code which should
// trigger an APZ scroll animation.
await promiseNativeMouseClickOnScrollbarTrack(anchor, xoffset, yoffset);
// Run 1000 frames, that should be enough to let the scroll animation start
// and run to completion. We check that it scrolled at least half the visible
// height, since we expect about a full screen height minus a few lines.
for (let i = 0; i < 1000; i++) {
await promiseOnlyApzControllerFlushed();
let pageScrollAmount = scroller.scrollTop;
ok(pageScrollAmount > scroller.clientHeight / 2,
`Scroll offset is ${pageScrollAmount}, should be near clientHeight ${scroller.clientHeight}`);
// Now we do two clicks in quick succession, but with a few frames in between
// to verify the scroll animation from the first click is active before the
// second click happens.
await promiseNativeMouseClickOnScrollbarTrack(anchor, xoffset, yoffset);
for (let i = 0; i < 5; i++) {
await promiseOnlyApzControllerFlushed();
let curPos = scroller.scrollTop;
ok(curPos > pageScrollAmount, `Scroll offset has increased to ${curPos}`);
ok(curPos < pageScrollAmount * 2, "Second page-scroll is not yet complete");
await promiseNativeMouseClickOnScrollbarTrack(anchor, xoffset, yoffset);
// Run to completion and check that we are around 3x pageScrollAmount, with
// some allowance for fractional rounding.
for (let i = 0; i < 1000; i++) {
await promiseOnlyApzControllerFlushed();
curPos = scroller.scrollTop;
ok(Math.abs(curPos - (pageScrollAmount * 3)) < 3,
`Final scroll offset ${curPos} is close to 3x${pageScrollAmount}`);
.then(subtestDone, subtestFailed);
<div style="position:fixed; bottom: 0; right: 0; width: 1px; height: 1px" id="anchor"></div>
<div style="height: 300vh; margin-bottom: 10000px; background-image: linear-gradient(red,blue)"></div>
The above div is sized to 3x screen height so the linear gradient is more steep in terms of
color/pixel. We only scroll a few pages worth so we don't need the gradient all the way down.
And then we use a bottom-margin to make the page really big so the scrollthumb is
relatively small, giving us lots of space to click on the scrolltrack.