measure
Measures this node on the owning tree thread.
The scope is valid only for the duration of this callback. Access after return or from another thread fails. If a callback or scope operation throws after pipeline work begins, the owning tree is poisoned and cleanup is attempted. This happens only when the exception escapes this callback.
Return
a size satisfying constraints.
Parameters
scope
the current direct-child measurement scope.
constraints
the size constraints for this node.