---
title: offsetHeight especially slow in Firefox for Mac
teaser: offsetHeight has performance problems on one browser.
tags: web,javascript,performance
author: Chad Pytel
published_on: 2006-10-14
---

At thoughtbot, we're working on an exciting piece of code that will someday be
shared with the rest of the world.  Suffice to say, its a very intensive bit of
JavaScript code that stresses the boundaries of all browsers.  After initially
solving a lot of the performance problems by offloading a lot of the
calculations to <abbr title="Cascading Style Sheets">CSS</abbr> instead of
JavaScript (lots of the elements on the page effect the position of all the
others), we were cruising along, only to hit a brick wall.

While performance in IE, Firefox on Windows, and Safari was acceptable (the
performance of Safari has been running circles around the other browsers),
performance on Firefox on Mac was incredibly poor.  Amazingly so.

After the initial panic, I set to tracking the precise cause of the performance
issues.  By commenting out large sections of the code, I was able to determine
that we were calling offsetHeight on some DIVs repeatedly each time an event
fired (and it fires a lot).

A quick Google search indicated that yes, some people have documented
performance issues with offsetHeight ([here][web-ref]).

[web-ref]: http://www.webreference.com/programming/javascript/mk/column2/3.html

While I can understand why offsetHeight is slow, I don't understand why the
performance of it in Firefox on Mac (Macbook Pro) was so much worse than any
other browser.