[css-sizing] Last remembered size when fragmented?

https://drafts.csswg.org/css-sizing-4/#last-remembered

record the current inner dimensions of its principal box as its last remembered size.

What if the principal box is fragmented e.g. into different columns when inside a multicol container?

ResizeObserver only reports the size in the 1st column: https://drafts.csswg.org/resize-observer-1/#resize-observer-entry-interface

the current definitions of content rect and border box do not mention how those boxes are affected by multi-column layout. In this spec, there will only be a single ResizeObserverSize returned in the FrozenArray, which will correspond to the dimensions of the first column. A future version of this spec will extend the returned FrozenArray to contain the per-fragment size information.

So that's what Blink stores as the last remembered size.

Note that the last rememebred size is only used when the element has size containment, which makes it monolithic. So storing the sizes per fragment doesn't really make sense.

Just storing the size of the 1st fragment may be reasonable enough, but this should be clarified.

Start with the CSS Sizing 4 section on the last remembered size and compare it with the Resize Observer definitions of content rect, border box, and fragment sizing. Determine how a fragmented principal box should be represented, then clarify the specification to state whether the first fragment or another size is used.