Greptile Summary

Adds a layoutAnchor prop to control the anchor point for relative layout projection, fixing child drift during parent layout animations in centered layouts (flexbox, grid). The implementation modifies calcRelativeAxis, calcRelativeAxisPosition, calcRelativeBox, and calcRelativePosition in delta-calc.ts to accept an optional anchor parameter, then threads this through all call sites in create-projection-node.ts. Setting layoutAnchor={false} disables relative projection entirely.

• Core math (delta-calc.ts): Uses mixNumber to interpolate the anchor point between parent.min and parent.max, with a fast path when anchor=0 that preserves existing behavior.
• Projection node (create-projection-node.ts): Passes anchor through to all calcRelativePosition/calcRelativeBox calls; gates relative target creation on layoutAnchor !== false. One call site (attemptToResolveRelativeTarget) is missing the layoutAnchor !== false guard — see inline comment.
• Types: layoutAnchor?: { x: number; y: number } | false added to both ProjectionNodeOptions and MotionNodeLayoutOptions.
• Tests: 8 new unit tests and 2 Cypress E2E tests with good coverage of backward compatibility and the centered-child use case.

Confidence Score: 3/5

• Generally safe to merge, but has a minor gap in the layoutAnchor={false} code path that should be addressed.
• The core anchor math is correct and well-tested with roundtrip verification. Default behavior is unchanged (backward compatible). However, the attemptToResolveRelativeTarget code path in create-projection-node.ts is missing the layoutAnchor !== false guard, which means layoutAnchor={false} doesn't fully disable relative projection in all scenarios. This is a logic gap rather than a crash-level bug, but it undermines the documented behavior of the false option.
• packages/motion-dom/src/projection/node/create-projection-node.ts — missing layoutAnchor !== false gate at line 1291

Important Files Changed

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["resolveTargetDelta()"] --> B{"targetDelta or relativeTarget?"}
    B -->|Neither| C{"layoutAnchor !== false?"}
    C -->|Yes| D{"relativeParent && parent.layout?"}
    C -->|No / false| E["removeRelativeTarget()"]
    D -->|Yes| F["createRelativeTarget(anchor)"]
    D -->|No| E
    B -->|relativeTarget exists| G["calcRelativeBox(target, relativeTarget, parent.target, anchor)"]
    B -->|targetDelta exists| H["applyBoxDelta(target, targetDelta)"]
    H --> I{"attemptToResolveRelativeTarget?"}
    I -->|Yes| J["createRelativeTarget(anchor)<br/><b>⚠ Missing layoutAnchor!==false gate</b>"]
    I -->|No| K["Continue"]

    style J fill:#fff3cd,stroke:#ffc107
    style C fill:#d4edda,stroke:#28a745

_{Last reviewed commit: bd5de2b}