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<!DOCTYPE html>
<link rel="help" href="https://drafts.csswg.org/css-scroll-snap-1/#scroll-snap-type" />
<title>Mouse-wheel scroll snapping speed</title>
<meta name="assert"
content="Test passes if mousewheel snaps without pausing">
<style>
#scroller {
scroll-snap-type: block mandatory;
overflow: scroll;
height: 400px;
width: 400px
}
#space {
width: 200px;
height: 4000px;
}
.box {
scroll-snap-align: start;
background: blue;
margin-bottom: 10px;
width: 100px;
height: 100px;
}
</style>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="/resources/testdriver.js"></script>
<script src="/resources/testdriver-vendor.js"></script>
<script src="/resources/testdriver-actions.js"></script>
<script src="/dom/events/scrolling/scroll_support.js"></script>
<script src="../support/common.js"></script>
<div id="scroller">
<div class="box"></div>
<div id="target" class="box"></div>
<div id="space"></div>
</div>
<script>
promise_test(async t => {
await waitForCompositorReady();
const scroller = document.getElementById("scroller");
scroller.scrollTo(0, 0);
assert_equals(scroller.scrollTop, 0, "verify test pre-condition");
const scrollTop = () => {
return scroller.scrollTop;
};
const scrollPromise = waitForScrollEvent(scroller);
const wheelPromise = waitForWheelEvent(scroller);
const targetBounds =
document.getElementById('target').getBoundingClientRect();
const dy = targetBounds.top / 2 + 1;
const actions = new test_driver.Actions()
.scroll(50, 50, 0, dy, {origin: scroller, duration: 100});
await actions.send();
await wheelPromise;
await scrollPromise;
// Detect first pause in scrolling. As expected to snap right at the end of
// the scroll, there should be no appreciable pause before the snap takes
// place. Once the scrolling settles, we are expected to be at the snap
// position.
const scrollStabilizedPromise = waitForAnimationEnd(scrollTop);
await scrollStabilizedPromise;
assert_approx_equals(scroller.scrollTop, 110, 0.5,
'Failed to advance to next snap target');
}, "Wheel-scroll triggers snap to target position without intermediate pause.");
</script>
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