[css-break] Clarify the expections of monolithic boxes that fits but has descendants which overflow #6056
Description
In https://chromium-review.googlesource.com/c/chromium/src/+/2696632 Morten Stenshorne says:
Avoid fragmentainer overflow from children of monolithic content.
I can't find any support for this interpretation of "monolithic" in the css-break spec. On the contrary, it only says that an element's own box should not be fragmented[1]. Technically, the box itself isn't fragmented when only its descendants overflow. To be clear, I'm not objecting to their interpretation, but the expected behavior of this case should probably be explicitly specified if we want interoperability for it.