.track {
  position: fixed;
  top: 0;
  left: 0;
  width: var(--track-width);
  height: 100vh;
  transform: translateX(0);
  will-change: transform;
}

.scroll-spacer {
  /* static/in-flow on purpose: must paint behind the fixed rail/track regardless of DOM order */
  /* height is set by JS to match the horizontal scroll distance */
}