Root cause: SortableContext registers each rack column as a droppable.
Each column is ~1800px tall (42U x 44px). The default closestCenter
algorithm compared center-to-center distances, so the rack column's
center consistently beat the 44px RackSlot's center — meaning over.data
resolved to { dragType: 'rack' } and handleDragEnd's check for
dropType === 'slot' never matched. Drops silently did nothing.
Fix: replace closestCenter with a two-phase collision detection:
1. pointerWithin — returns droppables whose bounding rect contains
the actual pointer position. Slots are exactly hit-tested.
2. closestCenter fallback — used when the pointer is not within any
registered droppable (e.g. dragging a rack header between columns
for sortable reorder where the pointer may be in the gap).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
jason
172896b85f