On my personal website, I've manually added an object to test this implementation, and although it works, the offset is off from the original scroll position, as seen here in this image

The element was appended to my page via developer tools with the html below.
<div id="filter" style="display:block;position:fixed;top:0;left:0;width:100%;height:200px;z-index:9999;margin:0;padding:0;border:1px solid red;box-sizing:border-box;"></div>

Then the script was ran to apply the changes and append the iframe inside that div.

I was able to remedy the issue using a manual dirty fix to take the backdropapply element's offsetHeight and multiplying that by two, and subtracting that to the scroll offset.

var scroll = this.scrollY
var scrollx = this.scrollX
scroll = scroll - (backdropapply.offsetHeight * 2);