Positions fixed doesn't work when using -webkit-transform

The CSS Transforms spec explains this behavior. Elements with transforms act as a containing block for fixed position descendants, so position:fixed under something with a transform no longer has fixed behavior.

They do work when applied to the same element; the element will be positioned as fixed, and then transformed.

After some research, there has been a bug report on the Chromium website about this issue, so far Webkit browsers can't render these two effects together at the same time.

I would suggest adding some Webkit only CSS into your stylesheet and making the transformed div an image and using it as the background.

@media screen and (-webkit-min-device-pixel-ratio:0) {
  /* Webkit-specific CSS here (Chrome and Safari) */

  #transformed_div {
    /* styles here, background image etc */
  }
}

So for now you'll have to do it the old fashioned way, until Webkit browsers catch up to FF.

EDIT: As of 10/24/2012 the bug has not been resolved.

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This appears to not be a bug, but an aspect of the specification due to the two effects requiring separate coordinate systems and stacking orders. As explained in this answer.

Something (a bit hacky) that worked for me is to position:sticky instead:

.fixed {
     position: sticky;
}