I have the following setup: (try scrolling the content)
border: 1px solid black;
The scrollable area is defined by the wrapper via its
overflow: auto declaration. Since wrapper serves as the containing block for both the content and the backdrop (due to
position: relative), this causes both elements to scroll together. In other words, this is due to both
overflow: auto and
position: relative on the same parent element, working in tandem.
Note that absolute positioning does not make an element immune to scrolling; when an absposed element does not appear to scroll, that is simply because its containing block does not scroll, and whatever is scrolling apart from the absposed element is some other element that is not its containing block. That is not the case in your setup. See the last example in section 11.1.1 of the spec for another example of this.