fractional-scroll-height-chaining.html

<!DOCTYPE html>
<script src="../../resources/gesture-util.js"></script>
<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<script src="../../resources/percent-based-util.js"></script>
<script src="../../resources/testdriver.js"></script>
<script src="../../resources/testdriver-actions.js"></script>
<script src="../../resources/testdriver-vendor.js"></script>
<style>

#scroller {
  background-color: #ccc;
  margin-top: 20px;
  overflow: scroll;
}

#fracheight {
  height: 1200.3px;
}

</style>
<div id="scroller">
  <div id="fracheight"></div>
</div>

<script>
function accumulateWheelScrollDelta() {
  return new Promise((resolve, reject) => {
    let scrollAmount = 0;
    const scrollendListener = () => {
      if (!scrollAmount) {
        reject('Expected wheel scroll');
      } else {
        resolve(scrollAmount);
      }
    };
    const wheelListener = (event) => {
      scrollAmount += event.deltaY;
    };
    document.addEventListener('scrollend', scrollendListener);
    document.addEventListener('wheel', wheelListener);
  });
}

promise_test(async () => {
    // Not seeing wheel events if the scrollAccumulator is initialized
    // immediately ahead of a wheel scroll. This likely has something to do with
    // how the events are queued and synchronously handled once the gesture
    // promise is resolved. As long as the wheel event listener is queued in an
    // earlier frame than the wheel scroll, events are properly triggered.
    const scrollAccumulator = accumulateWheelScrollDelta();
    await waitForCompositorCommit();
    const scroller = document.getElementById("scroller");
    const center = pointInElement(scroller, 50, 50);
    // Mouse click to ensure the element has focus before scrolling.
    await mouseClick(center.x, center.y);
    // Scroll 1 tick. The actual scroll amount may differ if percent-based
    // scrolling is enabled.
    const adjustedPixelsPerTick = calculateExpectedScroll(
      document.scrollingElement, 0, pixelsPerTick()).y;
    await wheelScroll(center.x, center.y, 0, adjustedPixelsPerTick, document);
    const scrollDelta = await scrollAccumulator;
    assert_approx_equals(window.scrollY, scrollDelta, 0.01);
    assert_approx_equals(window.scrollY, adjustedPixelsPerTick, 0.01);
}, "Verify that wheel scrolls chain correctly from a fractional-height scroller to its parent.");
</script>
chromium/third_party/blink/web_tests/fast/scrolling/fractional-scroll-height-chaining.html
