.box-anchor {
  display: inline-block;
  width: 0;
  height: 1em;
  vertical-align: baseline;
  pointer-events: none;
}

.box-overlay {
  position: absolute;
  pointer-events: none;
  z-index: 20;
  outline: 2px solid #ef4444;
  background: rgba(239, 68, 68, 0.12);
  box-sizing: border-box;
}

/* Dummy fragment that delays the overlay by one click after a line step. */
pre > .box-trigger {
  position: absolute;
  width: 0;
  height: 0;
  overflow: hidden;
  opacity: 0 !important;
  pointer-events: none;
}