Optimizing native hit testing of DOM elements (Chrome)

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Interesting, that pointer-events: none has no effect. But if you think about it, it makes sense, since elements with that flag set still obscure other elements’ pointer events, so the hittest has to take place anyways.

What you can do is put a overlay over critical content and respond to mouse-events on that overlay, let your code decide what to do with it.

This works because once the hittest algorithm has found a hit, and I’m assuming it does that downwards the z-index, it stops.

With overlay

// ================================================
// Increase or decrease this value for testing:
var NUMBER_OF_OBJECTS = 40000;
// Wether to use the overlay or the container directly
var USE_OVERLAY = true;
// ================================================

var overlay = document.getElementById("overlay");
var container = document.getElementById("container");
var contents = document.getElementById("contents");

for (var i = 0; i < NUMBER_OF_OBJECTS; i++) {
    var node = document.createElement("div");
    node.innerHtml = i;
    node.className = "node";
    node.style.top = Math.abs(Math.random() * 2000) + "px";
    node.style.left = Math.abs(Math.random() * 2000) + "px";
    contents.appendChild(node);
}

var posX = 100;
var posY = 100;
var previousX = null;
var previousY = null;

var mousedownHandler = function (e) {
    window.onmousemove = globalMousemoveHandler;
    window.onmouseup = globalMouseupHandler;
    previousX = e.clientX;
    previousY = e.clientY;
}

var globalMousemoveHandler = function (e) {
    posX += e.clientX - previousX;
    posY += e.clientY - previousY;
    previousX = e.clientX;
    previousY = e.clientY;
    contents.style.transform = "translate3d(" + posX + "px, " + posY + "px, 0)";
}

var globalMouseupHandler = function (e) {
    window.onmousemove = null;
    window.onmouseup = null;
    previousX = null;
    previousY = null;
}

if(USE_OVERLAY){
	overlay.onmousedown = mousedownHandler;
}else{
	overlay.style.display = 'none';
	container.onmousedown = mousedownHandler;
}


contents.style.transform = "translate3d(" + posX + "px, " + posY + "px, 0)";
#overlay{
  position: absolute;
  top: 0;
  left: 0;
  height: 400px;
  width: 800px;
  opacity: 0;
  z-index: 100;
  cursor: -webkit-grab;
  cursor: -moz-grab;
  cursor: grab;
  -moz-user-select: none;
  -ms-user-select: none;
  -webkit-user-select: none;
  user-select: none;
}

#container {
  height: 400px;
  width: 800px;
  background-color: #ccc;
  overflow: hidden;
}

#container:active {
  cursor: move;
  cursor: -webkit-grabbing;
  cursor: -moz-grabbing;
  cursor: grabbing;
}

.node {
  position: absolute;
  height: 20px;
  width: 20px;
  background-color: red;
  border-radius: 10px;
  pointer-events: none;
}
<div id="overlay"></div>
<div id="container">
    <div id="contents"></div>
</div>

Without overlay

// ================================================
// Increase or decrease this value for testing:
var NUMBER_OF_OBJECTS = 40000;
// Wether to use the overlay or the container directly
var USE_OVERLAY = false;
// ================================================

var overlay = document.getElementById("overlay");
var container = document.getElementById("container");
var contents = document.getElementById("contents");

for (var i = 0; i < NUMBER_OF_OBJECTS; i++) {
    var node = document.createElement("div");
    node.innerHtml = i;
    node.className = "node";
    node.style.top = Math.abs(Math.random() * 2000) + "px";
    node.style.left = Math.abs(Math.random() * 2000) + "px";
    contents.appendChild(node);
}

var posX = 100;
var posY = 100;
var previousX = null;
var previousY = null;

var mousedownHandler = function (e) {
    window.onmousemove = globalMousemoveHandler;
    window.onmouseup = globalMouseupHandler;
    previousX = e.clientX;
    previousY = e.clientY;
}

var globalMousemoveHandler = function (e) {
    posX += e.clientX - previousX;
    posY += e.clientY - previousY;
    previousX = e.clientX;
    previousY = e.clientY;
    contents.style.transform = "translate3d(" + posX + "px, " + posY + "px, 0)";
}

var globalMouseupHandler = function (e) {
    window.onmousemove = null;
    window.onmouseup = null;
    previousX = null;
    previousY = null;
}

if(USE_OVERLAY){
	overlay.onmousedown = mousedownHandler;
}else{
	overlay.style.display = 'none';
	container.onmousedown = mousedownHandler;
}


contents.style.transform = "translate3d(" + posX + "px, " + posY + "px, 0)";
#overlay{
  position: absolute;
  top: 0;
  left: 0;
  height: 400px;
  width: 800px;
  opacity: 0;
  z-index: 100;
  cursor: -webkit-grab;
  cursor: -moz-grab;
  cursor: grab;
  -moz-user-select: none;
  -ms-user-select: none;
  -webkit-user-select: none;
  user-select: none;
}

#container {
  height: 400px;
  width: 800px;
  background-color: #ccc;
  overflow: hidden;
}

#container:active {
  cursor: move;
  cursor: -webkit-grabbing;
  cursor: -moz-grabbing;
  cursor: grabbing;
}

.node {
  position: absolute;
  height: 20px;
  width: 20px;
  background-color: red;
  border-radius: 10px;
  pointer-events: none;
}
<div id="overlay"></div>
<div id="container">
    <div id="contents"></div>
</div>

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