childTransform
Returns the additional transform for one placed direct child.
The runtime invokes this method on the owning tree thread only after successful layout and supplies a valid placed direct-child index. Implementations retain ownership of their state; the returned immutable value may be retained by the runtime until layout is invalidated.
Return
finite immutable transform combined with ordinary child placement.
Parameters
index
valid direct-child index in the current placed child set.
Throws
when resolving the transform fails; the exact failure propagates through the active tree operation.