function validateHoverState(elementList, hoverIndex, hoverMismatch) {
  for (let i = 0; i < elementList.length; i++) {
    if (elementList[i].matches(':hover') != (i == hoverIndex)) {
      hoverMismatch();
    }
  }
}

function elementHeight() {
  let height = undefined;
  document.querySelectorAll('div').forEach((div) => {
    if (height === undefined) {
      height = getComputedStyle(div).height;
    } else {
      if (height !== getComputedStyle(div).height) {
        throw new Error("Test requires all 'divs' to have the same height");
      }
    }
  });
  return parseInt(height);
}

function runHoverStateOnScrollTest(scrollCallback, targetIndex) {
  verifyTestDriverLoaded();
  const runTest = async (resolve, reject) => {
    await waitForCompositorReady();

    const array = document.getElementsByClassName('hoverme');
    const center = elementCenter(array[0]);
    // Move cursor to 1st element.
    await mouseClick(center.x, center.y);
    assert_equals(document.scrollingElement.scrollTop, 0);
    validateHoverState(array, 0, () => {
      reject('Not hovering over the first element');
    });

    // While scrolling the hover state shouldn't change. Note, this
    // behavior is not addressed in the specs. The rationale is to prevent the
    // scroll from potentially affecting layout.
    let firstHoverUpdate = undefined;
    const hoverStateCheck = () => {
      validateHoverState(array, 0, () => {
        if (!firstHoverUpdate) {
          firstHoverUpdate = document.scrollingElement.scrollTop;
        }
      });
    };

    const scrollListener =
        document.addEventListener('scroll', hoverStateCheck);

    await scrollCallback(center.x, center.y);

    // If the hover change occurs after the last scroll event then
    // firstHoverUpdate remains undefined.  If the last scroll event is delayed
    // such that it occurs after the hover state update, that is OK provided
    // that we have reached the end of the scroll.
    if (firstHoverUpdate &&
        firstHoverUpdate != document.scrollingElement.scrollTop) {
      reject('Unexpected change to hover state during scroll');
    }

    document.removeEventListener('scroll', scrollListener);

    await waitForCompositorReady();

    // Once scrolling is complete, the hover state must update.
    validateHoverState(array, targetIndex, () => {
      reject('Hover state must be updated after scroll ends');
    });
    resolve();
  };
  return new Promise((resolve, reject) => {
    requestAnimationFrame(() => { runTest(resolve, reject); });
  });
}