measure

Lay this child out under constraints and report the size it took.

May be called more than once per pass. CSS Flexbox §9.7 resolves flexible lengths by measuring a child at its flex basis, distributing free space, and measuring again at the resolved size, so a flexbox written on this API necessarily measures twice.

Each call lays the child out under its constraints, and the Placeable it returns carries that layout: whichever placeable the container places is what mounts, so measuring a child once to probe it and again to place it costs the probe and nothing more.

The child's own size props — width, min and max, aspect ratio, margins, padding and border — apply within constraints, and the placeable reports the size it took, kept within them.

Not from the placement block: placing only chooses among the measurements already made. Not from an intrinsic override either, which answers a number.