css/css-scroll-snap/snap-after-initial-layout/initial-snap-matches-scroll.html

<!DOCTYPE html>
<link rel="help" href="https://drafts.csswg.org/css-scroll-snap-1/#scroll-snap-align">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="../support/common.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>
<style>
.scroller {
  height: 150px;
  width: 300px;
  overflow: auto;
  position: relative;
  white-space: nowrap;
  scroll-snap-type: x mandatory;
}
.padding {
  padding: 0 200px;
}
.child {
  scroll-snap-align: center;
  display: inline-block;
  width: 201px;
  height: 100%;
  margin: 0 200px;
  background-color: blue;
}
</style>

<div class="scroller" id="scroller1">
  <div class="padding">
    <div class="child"></div>
    <div class="child"></div>
  </div>
</div>

<div class="scroller" id="scroller2">
  <div class="padding">
    <div class="child"></div>
    <div class="child"></div>
  </div>
</div>

<script>
promise_test(async () => {

  // Scroll to the second child.
  let scrollEndPromise = waitForScrollEndFallbackToDelayWithoutScrollEvent(scroller2);
  scroller2.scrollTo(scroller2.scrollWidth, 0);
  await scrollEndPromise;

  assert_greater_than(scroller2.scrollLeft, scroller1.scrollLeft);

  // Scroll back to the first child.
  scrollEndPromise = waitForScrollEndFallbackToDelayWithoutScrollEvent(scroller2);
  await keyPress(scroller2, "ArrowLeft");
  await scrollEndPromise;

  // After scrolling to snap to the same child, both scrollers should have the
  // same scroll position.
  assert_equals(scroller2.scrollLeft, scroller1.scrollLeft);
}, "initial scroll snap matches scrolling scroll snap position.");
</script>