You should not add a shadow to the ::before in this case, add to it's parents. Also I decrease the spread-radius of the shadow.

// styles.css

.App {
  font-family: sans-serif;
  text-align: center;
}

.ReactVirtualized__Grid__innerScrollContainer:first-of-type::before {
  content: "";
  position: absolute;
  top: 0;
  right: 0;
  transform: translateX(100%);
  width: 8px;
  height: 100%;
  pointer-events: none;
  z-index: 1;
}

.BottomLeftGrid_ScrollWrapper:first-of-type,
.ReactVirtualized__Grid:first-of-type {
  box-shadow: 12px 0px 8px -4px rgba(0, 0, 0, 0.25);
}