OverlayScrollbars/packages/overlayscrollbars/src/styles/sizeobserver.scss

$scrollbar-cushion: 100px;
$inflate-margin: 200px;

.os-size-observer,
.os-size-observer-listener {
  direction: inherit;
  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 {
  writing-mode: horizontal-tb;
  position: absolute;
  left: 0;
  top: 0;
}

.os-size-observer {
  height: 25%;
  width: 25%;
  z-index: -1;
  contain: strict;
  display: flex;
  flex-direction: column;
  flex-wrap: nowrap;
  padding: inherit;
  border: inherit;
  box-sizing: inherit;
  margin: 0;

  &::before,
  &::after {
    content: '';
    flex: none;
    box-sizing: inherit;
  }
  &::before {
    padding: 1px;
    width: 1px;
    height: 1px;
  }
  &::after {
    content: '';
    padding: inherit;
    border: inherit;
  }
}

.os-size-observer-appear {
  // "forwards" is important for older browsers
  animation: os-size-observer-appear-animation 1ms forwards;
}

.os-size-observer-listener {
  box-sizing: border-box;
  position: relative;
  flex: auto;
  margin: -#{$inflate-margin};

  &.ltr {
    margin-right: -#{$inflate-margin * 2};
    margin-left: 0;
  }
  &.rtl {
    margin-left: -#{$inflate-margin * 2};
    margin-right: 0;
  }

  // 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;
  }
}