ElementFromPoint() In Gecko

I recently had need for the method document.elementFromPoint() found in Internet Explorer (http://msdn.microsoft.com/workshop/author/dhtml/reference/methods/elementfrompoint.asp).

My immediate method for implementing this was artificially creating an arbitrary mouse event (decided on mousemove) at the specified coordinates, setting up an event listener on the document, firing the event, record the target, and remove the listener.

However, I apparently didn't pay close enough attention to this (http://www.w3.org/TR/2000/REC-DOM-Level-2-Events-20001113/events.html#Events-EventTarget-dispatchEvent):

The target of the event is the EventTarget on which dispatchEvent is called.

However, I had a hunch that Mozilla was merely redirecting the event to the document to be correct, and actually stored somewhere the "correct" target. After dumping the event object, I found a property which must have been recently introduced (Mozilla 1.4 branch apparently) called "explicitOriginalTarget" which is like originalTarget, but apparently never targets anonymous content, and in this case, also differs by referring to the element where the coordinates of the mouse event were.

Anyhoo, here's some code. I've tested it in various Mozilla 1.4 builds successfully (and double checked explicitOriginalTarget in < 1.4 builds to find that it doesn't exist). Firebird 0.6 is also based on 1.4, so it works in that too. By using the browser's box object and mouse events, instead of hacking my own, I don't think there should be any issues with it picking up elements that can't be seen.


// Program: document.elementFromPoint(int clientX, int clientY) in Gecko
// Author: Jason Karl Davis (www.jasonkarldavis.com)
// Date: 15 June 2003
// Purpose: Emulate Internet Explorer's document.elementFromPoint method as described here:
// http://msdn.microsoft.com/workshop/author/dhtml/reference/methods/elementfrompoint.asp
// Requirements: A browser built off of the 1.4 branch of Mozilla (or better)
// Distribution: You may freely distribute and use this script as long as these comments remain intact

if (navigator.product == "Gecko") {
Document.prototype.elementFromPoint = function(x, y) {
this.addEventListener("mousemove", this.elementFromPoint__handler, false);
var event = this.createEvent("MouseEvents");
var box = this.getBoxObjectFor(this.documentElement);
var screenDelta = { x: box.screenX, y: box.screenY };
event.initMouseEvent("mousemove", true, false, this.defaultView, 0,
x + screenDelta.x, y + screenDelta.y, x, y,
false, false, false, false, 0, null);
this.dispatchEvent(event);
this.removeEventListener("mousemove", this.elementFromPoint__handler, false);
return this.elementFromPoint__target;
}
Document.prototype.elementFromPoint__handler = function (event) {
this.elementFromPoint__target = event.explicitOriginalTarget;

// reparent target if it is a text node to emulate IE's behavior
if (this.elementFromPoint__target.nodeType == Node.TEXT_NODE)
this.elementFromPoint__target = this.elementFromPoint__target.parentNode;

// change an HTML target to a BODY target to emulate IE's behavior (if we are in an HTML document)
if (this.elementFromPoint__target.nodeName.toUpperCase() == "HTML" && this.documentElement.nodeName.toUpperCase() == "HTML")
this.elementFromPoint__target = this.getElementsByTagName("BODY").item(0);

event.preventDefault();
event.stopPropagation();
}
Document.prototype.elementFromPoint__target = null;
}


And the reason I prototyped Document instead of adding it to document directly is that any extra documents you may have loaded on the same page inherit those methods. For example, an iframe or something. And it /should/ be usable in other, non-HTML documents as well, such as MathML, SVG, XUL, etc. :)