Remaining todo items:

• introduce LeafTransform3D with all the associated ripple effect [...] New LeafTransform3D, replacing OutOfTreeTransform3D #7015
• finish removal of legacy types (or deprecate where appropriate) Remove Scale3D/Transform3D/TranslationRotationScale3D datatypes, remove Transform3D component #7000
• deprecate (or remove for new stuff) stray rotation components from archetypes and add out of tree support there Use LeafTranslation (centers), LeafRotationQuat and LeafRotationAxisAngle directly on Boxes3D/Ellipsoids3D #7029
  • box3d
  • ellipsoids (remove!)
  • what else?
• remove old Rotation3D component Removed now unused Rotation3D component & datatype #7030
• use LeafTransform3D on Box3D & Arrows3D & Ellipsoids instead (don't forget to point out transform relationship!) Use LeafTranslation (centers), LeafRotationQuat and LeafRotationAxisAngle directly on Boxes3D/Ellipsoids3D #7029
• extra pass over migration guide - is all correct, is something missing

• Split out of Stop using arrow unions #6388
• Related to
  • Transform3D should have an affine3d= helper. #3645
  • Add RotationMatrix similar to Quaternion and RotationAxisAngle #2868
  • Introduce new enum component to express from_parent #6863

// Two possibilities:
// - Only legal to set one of them
// - Or apply them all in deterministic order
archetype Transform {
    mat4: Option<Mat4>,
    translation: Option<Translation3>,
    mat3: Option<Mat3>,
    rotation: Option<Rotation3D>,
    scale3: Option<Scale3D>,
    scale: Option<Scale>,
}

Old proposal

How to handle OutOfTreeTransform:

Introduce a new boolean component that coerces a transform batch to be out of tree. OutOfTreeTransform { enabled: bool }
if one encounters several several parent transforms and if they have the OutOfTreeTransform == false the viewer issues a warning and resolves in a best effort manner

To consider: The fallback provider for OutOfTreeTransform can be clever and look at transform counts, automatically assuming true if there's several transforms.

Drawback: Can no longer have out of tree transform side by side with in-tree transforms