elementFromPoint returns null after scrolling the page

Question

I have a javascript bookmarklet I put together to make an arduous task a little more bearable. Essentially I am going through hundreds of pages of training material and making sure that all of it has been properly swapped from Helvetica to Arial. The bookmarklet code is below, but a quick breakdown is that it creates a mousemove event listener and a small, absolutely positioned div. On mousemove events, the div moves to the new mouse position (offset by 10px down and right), gets the element under the mouse with elementFromPoint and shows the font-family property for that element. oh and it changes it's background color based on whether Arial appears within the property.

var bodyEl=document.getElementsByTagName("body")[0];
var displayDiv=document.createElement("div");
displayDiv.style.position="absolute";
displayDiv.style.top="0px";
displayDiv.style.top="0px";
bodyEl.appendChild(displayDiv);


function getStyle(el,styleProp) {
  var camelize = function (str) {
    return str.replace(/\-(\w)/g, function(str, letter){
      return letter.toUpperCase();
    });
  };

  if (el.currentStyle) {
    return el.currentStyle[camelize(styleProp)];
  } else if (document.defaultView && document.defaultView.getComputedStyle) {
    return document.defaultView.getComputedStyle(el,null)
                               .getPropertyValue(styleProp);
  } else {
    return el.style[camelize(styleProp)]; 
  }
}
function getTheElement(x,y) {return document.elementFromPoint(x,y);}

fn_displayFont=function displayFont(e) {
    e = e || window.event;
    var divX=e.pageX+10;
    var divY=e.pageY+10;
    var font=getStyle(getTheElement(e.pageX,e.pageY),"font-family");
    if (font.toLowerCase().indexOf("arial") != -1) {
        displayDiv.style.backgroundColor = "green";
    } else {
        displayDiv.style.backgroundColor = "red";
    }
    displayDiv.style.top= divY.toString() + "px";
    displayDiv.style.left= divX.toString() + "px";
    displayDiv.style.fontFamily=font;
    displayDiv.innerHTML=font;
}

window.addEventListener('mousemove', fn_displayFont);
document.onkeydown = function(evt) {
    evt = evt || window.event;
    if (evt.keyCode == 27) {
        window.removeEventListener('mousemove', fn_displayFont);
        bodyEl.removeChild(displayDiv);
    }
};

(for the record, I stole the style determining code from an answer here on SO, but I lost the tab not long after. Thanks, anonymous internet guy!) So this all works great - UNTIL I try to hover over a part of the page that is scrolled down from the top. The div sits at where it would be if I had the mouse on the very bottom of the screen while scrolled to the top of the page, and if I scroll down far enough firebug starts logging that e.pageX is undefined.

Any ideas?

Answer 1

Alrighty then, figured it out. I saw http://www.daniweb.com/web-development/javascript-dhtml-ajax/threads/276742/elementfrompoint-problems-when-window-has-been-scrolled- and thought it meant I had to minus the pageoffset straight away from the e.pageX/Y values, before I used it to calculate the div position or anything else, this just broke everything for me so I assumed it must have been unrelated - not so! From what I now understand the elementFromPoint method takes a point relative in the current view of the browser, which is to say, base on the top left corner of what can currently be seen, not the page as a whole. I fixed it by just taking the offset from the X and Y values when I was getting the element. The now-working code is below.

var bodyEl=document.getElementsByTagName("body")[0];
var displayDiv=document.createElement("div");
displayDiv.style.position="absolute";
displayDiv.style.top="0px";
displayDiv.style.top="0px";
bodyEl.appendChild(displayDiv);


function getStyle(el,styleProp) {
  var camelize = function (str) {
    return str.replace(/\-(\w)/g, function(str, letter){
      return letter.toUpperCase();
    });
  };

  if (el.currentStyle) {
    return el.currentStyle[camelize(styleProp)];
  } else if (document.defaultView && document.defaultView.getComputedStyle) {
    return document.defaultView.getComputedStyle(el,null)
                               .getPropertyValue(styleProp);
  } else {
    return el.style[camelize(styleProp)]; 
  }
}
function getTheElement(x,y) {return document.elementFromPoint(x,y);}

fn_displayFont=function displayFont(e) {
    e = e || window.event;
    var divX=e.pageX + 10;
    var divY=e.pageY + 10;
    var font=getStyle(getTheElement(e.pageX - window.pageXOffset,e.pageY - window.pageYOffset),"font-family");
    if (font.toLowerCase().indexOf("arial") != -1) {
        displayDiv.style.backgroundColor = "green";
    } else {
        displayDiv.style.backgroundColor = "red";
    }
    displayDiv.style.top= divY.toString() + "px";
    displayDiv.style.left= divX.toString() + "px";
    displayDiv.style.fontFamily=font;
    displayDiv.innerHTML=font;
}

document.addEventListener('mousemove', fn_displayFont);
document.onkeydown = function(evt) {
    evt = evt || window.event;
    if (evt.keyCode == 27) {
        window.removeEventListener('mousemove', fn_displayFont);
        bodyEl.removeChild(displayDiv);
    }
};