[css-containment] layout containment and overflow

Because an absolutely positioned elements sticking out of a layout-contained element may cause a scrollbar to appear on a containing element, there can still be layout changes out of the containing element, and that's an issue (tracked as issue 2 in the spec).

As suggested by the spec, one way out of this is for layout containment to imply paint containment as well, but there's an less heavy handed alternative. We could add one entry to the "Giving an element layout containment has the following effects:" list:

* If the computed value of the 'overflow-x' and 'overflow-y' properties of the containing element is ''visible'', any overflow out of the containing element is treated as ink overflow.

- Florian