Overleaf/services/web/public/js/libs/pdfListView/TextLayer.css

.plv-page-view {
  position: relative;
}
.plv-text-layer {
  position: absolute;
  left: 0;
  top: 0;
  right: 0;
  bottom: 0;
  color: #000;
  font-family: sans-serif;
  overflow: hidden;
  pointer-events: none;
}
.plv-text-layer > div {
  color: transparent;
  position: absolute;
  line-height: 1;
  white-space: pre;
  cursor: text;
  pointer-events: auto;
}
.plv-text-layer > div::selection {
  background:rgba(0,0,255,0.3);
}
.plv-text-layer > div::-moz-selection {
  background:rgba(0,0,255,0.3);
}