Read-only view of the widget tree’s geometry passed to
Widget::after_paint. Wraps a borrow of
the arena so a parent widget can read the layout-resolved bounds of
any descendant (typically by ids it memoised during build) once
their paint pass has committed.
A child that is either pre-registered (ID) or waiting to be inserted.
Used by the inline child() builder pattern: deferred children are stored
inside the container and resolved recursively when BuildContext::add()
inserts the container into the arena.
The main (distribution) axis of the linear stack currently negotiating a
child’s layout, if any. Set by HStack/VStack while they query children
so an orientation-agnostic flexible child (notably Spacer) can place its
minimum length on the main axis only and report 0 on the cross axis —
otherwise it imposes a spurious cross-axis floor on the stack. None when
the parent is not a linear stack (or in test contexts).