function assertPageOffset(expectedX, expectedY, el) {
  const offsetCoordinate = style.getPageOffset(el);
  assertEquals('x-coord page offset is wrong.', expectedX, offsetCoordinate.x);
  assertEquals('y-coord page offset is wrong.', expectedY, offsetCoordinate.y);
}
Prefabricated popup element for convenient. This is created during
setUp and is not attached to the document at the beginning of the
test.
@type {Element}