.scrollTop, 300);
  assert_equals(document_scroller.scrollTop, 500);

  // Create new snap container and append thedocument-level snap area as its
  // child.
  const inserted_scroller = document.createElement("div");
  inserted_scroller.id = "inserted-snap-container";
  const space = document.createElement("div");
  space.classList.add("space");
  inserted_scroller.appendChild(space);
  inserted_scroller.appendChild(document.getElementById("document-snap-area"));
  document_scroller.appendChild(inserted_sc