JSFiddle - React, Tailwind, and code Playground

HTML

<div id="box-with-shadow">
</div>

<div id="status">
</div>

CSS

body {
  margin: 0;
  padding: 0;
}

#box-with-shadow {
  display: block;
  width: 10px;
  height: 10px;
  box-shadow: 100px 130px #000, 90px 140px #000, 100px 140px #000, 110px 140px #000, 80px 150px #000, 90px 150px #000, 110px 150px #000, 120px 150px #000, 90px 160px #000, 100px 160px #000, 110px 160px #000, 100px 170px #000;
}

#status {
  width: 100%;
  height: 2em;
  background: red;
}

JavaScript

var boxWidth = parseInt($("#box-with-shadow").css('width'), 10);
var boxHeight = parseInt($("#box-with-shadow").css('height'), 10);
var boxOffsetX = parseInt($("#box-with-shadow").css('margin-left'), 10);
var boxOffsetY = parseInt($("#box-with-shadow").css('margin-top'), 10);
var boxShadowString = $('#box-with-shadow').css('box-shadow');
var boxShadows = boxShadowString.split(/,(?![^\(]*\))/);

// key: x-pos of the shadow, value: concatenated y-positions divided by comma
var keyValuePairs = fillKeyValuePairs(); 

var cursorX;
var cursorY;

document.onmousemove = function(e) {
  cursorX = e.pageX;
  cursorY = e.pageY;
  checkCursor();
}

function checkCursor() {
  var shadowHovered = false;
  for (i = cursorX - boxWidth; i <= cursorX && !shadowHovered; i++) {
    if (keyValuePairs[i] != null) {
      // At this point we know that somewhere on this x-position there is a shadow.
      // Now check if there is an accosiated y-interval for the found x-position
      for (j = cursorY - boxHeight; j <= cursorY; j++) {
        if ((keyValuePairs[i] + "").split(",").indexOf((j + "")) != -1) {
          shadowHovered = true;
          break;
        }
      }
    }
  }
  if (shadowHovered) {
    $("#status").css("background", "green");
    $("#status").text("Found shadow: xOffset = " + (i - 1) + "px, yOffset = " + j + "px");
  } else {
    $("#status").css("background", "red");
    $("#status").text("");
  }
}

function fillKeyValuePairs() {
  var keyValuePairs = [];

  for (index = 0; index < boxShadows.length; index++) {
    var xPos = parseInt(boxShadows[index].trim().match(/[0-9]+px/g)[0], 10) + boxOffsetX;
    var yPos = parseInt(boxShadows[index].trim().match(/[0-9]+px/g)[1], 10) + boxOffsetY;

    keyValuePairs[xPos] = keyValuePairs[xPos] != null ? keyValuePairs[xPos] + "," + yPos : yPos;
  }

  return keyValuePairs;
}