Greptile Summary

This PR fixes a long-standing bug (#3061) where Reorder.Group's onReorder callback only returned the measured (rendered) items — silently dropping any items that were outside the viewport in a virtualized list.

What changed:

• Group.tsx — updateOrder no longer maps the internal order array back to values and filters against values. Instead it detects the single pairwise swap that checkReorder produced (by finding the first position where order[i] and newOrder[i] differ), then applies that same swap directly to a copy of the full values prop. This preserves every item, measured or not.
• index.test.tsx — A new unit test renders a Reorder.Group with 5 values but only registers 3 of them (simulating offscreen items in a virtualized list), triggers a drag past the threshold, and asserts that onReorder returns all 5 values with the correct swap applied.

Key considerations:

• The swap-detection logic is correct given that checkReorder always delegates to moveItem(order, index, index ± 1), which for adjacent indices is always a simple pairwise swap. The approach would need revisiting if checkReorder were ever extended to support multi-item moves.
• values.indexOf uses reference equality, which is consistent with the rest of the Reorder API but means duplicate primitive values or re-created object references would not behave correctly — this is a pre-existing limitation, not introduced here.
• There is a minor robustness gap: if indexOf returns -1 (e.g., a registered item whose value has already been removed from values), the swap silently becomes a no-op rather than throwing or logging a warning.

Confidence Score: 4/5

• This PR is safe to merge; the fix is logically correct and well-tested, with one minor robustness gap around missing values.
• The swap-detection algorithm is provably correct given checkReorder's adjacent-only move behaviour (verified by reading moveItem). The new unit test exercises the exact virtualized-list scenario described in the bug. The only concern — a silent no-op when indexOf returns -1 — is a pre-existing edge case that is unlikely to surface in practice and does not regress existing behaviour.
• The swap logic in packages/framer-motion/src/components/Reorder/Group.tsx lines 129–133 should be reviewed for the silent indexOf === -1 edge case.

Important Files Changed

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["updateOrder called"] --> B{isReordering?}
    B -- yes --> Z[return early]
    B -- no --> C["checkReorder - order, item, offset, velocity"]
    C --> D{newOrder !== order?}
    D -- no change --> Z
    D -- swap detected --> E["isReordering = true"]
    E --> F["newValues = spread of full values prop"]
    F --> G["Loop over newOrder positions"]
    G --> H{Values differ at position i?}
    H -- no --> G
    H -- yes --> I["Find index a of old value in values\nFind index b of new value in values"]
    I --> J["Swap newValues at a and b"]
    J --> K[break]
    K --> L["onReorder called with full newValues array"]

    style L fill:#d4edda,stroke:#28a745
    style Z fill:#f8d7da,stroke:#dc3545

_{Last reviewed commit: c3c3c7f}