Resize events only work if iframe in viewport

The solution of `top:-100%;` and `margin:1px 0 0;` does not keep the iframe positioned in a viewable area at all times. If the target element is nested in an element with `overflow: hidden` and the target element has a negative top margin or a negative absolute positioned top the resize events stop working.

`top: 0; z-index: -1` seems to fix the issue but I'm not sure what all the consequences are.