OverlayScrollbars/packages/overlayscrollbars/src/sizeobserver.scss

$scrollbar-cushion: 100px;

.os-size-observer,
.os-size-observer-listener {
  direction: inherit;
  padding: inherit;
  border: inherit;
  margin: 0;
  pointer-events: none;
  overflow: hidden;
  visibility: hidden;
  box-sizing: border-box;
}

.os-size-observer,
.os-size-observer-listener,
.os-size-observer-listener-item,
.os-size-observer-listener-item-final {
  position: absolute;
  left: 0;
  top: 0;
}

.os-size-observer {
  height: 100%;
  width: 100%;
  z-index: -1;
}

.os-size-observer-appear {
  animation-duration: 0.001s;
  animation-name: os-size-observer-appear-animation;
}

.os-size-observer-listener {
  display: block;
  height: 500%;
  width: 500%;

  // lets assume no scrollbar is 100px wide
  & > .os-size-observer-listener-item {
    top: -$scrollbar-cushion;
    right: -$scrollbar-cushion;
    bottom: -$scrollbar-cushion;
    left: -$scrollbar-cushion;
  }
}

.os-size-observer-listener-scroll {
  box-sizing: content-box;
}

.os-size-observer-listener-item {
  right: 0;
  bottom: 0;
  overflow: hidden;
  direction: ltr;
  flex: none;
}

.os-size-observer-listener-item-final {
  transition: none;
}

@keyframes os-size-observer-appear-animation {
  from {
    z-index: 0;
  }
  to {
    z-index: -1;
  }
}