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.

Start by reproducing the resize-event failure with the hidden iframe nested inside an overflow-hidden element and positioned with a negative top offset. Compare that behavior with top: 0 and z-index: -1, checking that resize events continue to fire without introducing positioning or visibility regressions; done means the nested case remains observable.

Written by the indexing model from the issue text.