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Flutter box-constraint layout model

Document status: reviewed. Canonical source.

Summary

Flutter’s box layout passes constraints from parent to child, returns sizes from child to parent and assigns child positions in the parent. The standard box layout is one pass and each render object chooses a size within its incoming constraints.

Evidence

  • The Flutter constraint guide states the processing order as constraints down, sizes up and parent-assigned positions. Width and height requests can be constrained or ignored by ancestors. https://docs.flutter.dev/ui/layout/constraints (retrieved 2026-08-29).
  • The guide identifies the one-pass limitation: a box can choose only within parent constraints, does not choose its global position and cannot determine geometry independently of the tree. https://docs.flutter.dev/ui/layout/constraints#limitations (retrieved 2026-08-29).

NUIF relevance

Borrow BoxConstraints-compatible min/max sizing, row/column flex and parent-relative placement for a bounded lowering profile.

Adapt NUIF entities into a generated, profile-owned Dart widget subset. Stable identity metadata and source spans are required for reconciliation. Conformance requires a pinned Flutter engine, platform, pixel ratio and fonts.

Reject arbitrary Dart widget-tree import. Builders, state, inherited widgets, custom render objects, assets and platform plugins are executable semantics outside profile zero.