Modified Adwaita overshoot settings, no visible under- or overshoot.

gtk.css

.overshoot.top {
  background-image: none;
  background-size: 100% 5%, 100% 100%;
  background-repeat: no-repeat;
  background-position: center top;
  background-color: transparent;
  border: none;
  box-shadow: none; }
  .overshoot.top:backdrop {
    background-image: none;
    background-size: 100% 5%;
    background-repeat: no-repeat;
    background-position: center top;
    background-color: transparent;
    border: none;
    box-shadow: none; }
.overshoot.bottom {
  background-image: none;
  background-size: 100% 5%, 100% 100%;
  background-repeat: no-repeat;
  background-position: center bottom;
  background-color: transparent;
  border: none;
  box-shadow: none; }
  .overshoot.bottom:backdrop {
    background-image: none;
    background-size: 100% 5%;
    background-repeat: no-repeat;
    background-position: center bottom;
    background-color: transparent;
    border: none;
    box-shadow: none; }
.overshoot.left {
  background-image: none;
  background-size: 5% 100%, 100% 100%;
  background-repeat: no-repeat;
  background-position: left center;
  background-color: transparent;
  border: none;
  box-shadow: none; }
  .overshoot.left:backdrop {
    background-image: none;
    background-size: 5% 100%;
    background-repeat: no-repeat;
    background-position: left center;
    background-color: transparent;
    border: none;
    box-shadow: none; }
.overshoot.right {
  background-image: none;
  background-size: 5% 100%, 100% 100%;
  background-repeat: no-repeat;
  background-position: right center;
  background-color: transparent;
  border: none;
  box-shadow: none; }
  .overshoot.right:backdrop {
    background-image: none;
    background-size: 5% 100%;
    background-repeat: no-repeat;
    background-position: right center;
    background-color: transparent;
    border: none;
    box-shadow: none; }
    
.undershoot.top {
  background-color: transparent;
  background-image: none;
  padding-top: 1px;
  background-size: 10px 1px;
  background-repeat: repeat-x;
  background-origin: content-box;
  background-position: center top; }
.undershoot.bottom {
  background-color: transparent;
  background-image: none;
  padding-bottom: 1px;
  background-size: 10px 1px;
  background-repeat: repeat-x;
  background-origin: content-box;
  background-position: center bottom; }
.undershoot.left {
  background-color: transparent;
  background-image: none;
  padding-left: 1px;
  background-size: 1px 10px;
  background-repeat: repeat-y;
  background-origin: content-box;
  background-position: left center; }
.undershoot.right {
  background-color: transparent;
  background-image: none;
  padding-right: 1px;
  background-size: 1px 10px;
  background-repeat: repeat-y;
  background-origin: content-box;
  background-position: right center; }