[css-overflow] handling of negative scrollable overflow region is incorrect

[dbaron]

This testcase shows that the handling of the "negative scrollable overflow region" in the rules for computing scrollable overflow is interoperably a different behavior from the one the spec describes.

The spec describes that the union used to compute the scrollable area ignores boxes whose border box is entirely within the negative scrollable overflow region. However, the area that matters should be the scrollable overflow of the box (when applied), not the border box.

According to the spec the above testcase should not be scrollable, but it is interoperably scrollable across Chrome, Firefox, and Safari.

[bfgeek]

I think broadly speaking the spec needs to be reworked to define scrollable overflow in terms of rectangles being propagated, which would match how implementations work, and help sorting out these types of issues.