Absolute positioned elements not taken into account for screenshot dimension

With the issue for geckodriver filed by @juangj already 2 years ago we still have the problem in how to determine the correct screenshot dimension when elements have an absolute position, which means that they can be located outside of the documentElement's bounds.

In chapter 17. Screen capture, section 17.1 Take Screenshot:

Let root rect be the current top-level browsing context’s document element’s rectangle

Given the spec we never would be able to get those elements drawn as part of the screenshot. And in some cases the root element can have a zero height, which means the result is a empty screenshot with only some pixels in height.

The example as given on the other issue is:

<div style="position: absolute; width: 200px; height: 200px; background-color: blue;"></div>

Start with the WebDriver specification's section 17.1, Take Screenshot, and compare its document-element rectangle rule with the linked geckodriver issue and the absolute-positioned HTML example. Determine and document the intended screenshot bounds for content outside the document element, including zero-height roots; completion requires an agreed specification change.