handleDrop and handleDragOver closed over dragIndex/dropIndex state
values. When the last dragover and drop events fire in the same React
render cycle, the drop handler reads stale state (often null), causing
computeDropTarget to return null and skip the reorder.
Mirror dragIndex/dropIndex into refs that are updated synchronously
alongside setState. Event handlers now read from refs, ensuring they
always see the latest values regardless of React's batching schedule.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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