Visual gate: moving an item within a bag reshuffled every item ("the order is
not set"). Root cause: insert-before set ONLY the dragged item's ContainerSlot
to N, colliding with the item already at N; the sort-by-slot tie then reordered
the whole grid on every repaint. Fix: MoveItemOptimistic now does a proper
index-based INSERT (shift the others, renumber 0..N-1 gapless) like retail's
ItemList_InsertItem, and HandleDropRelease uses the target's GRID INDEX
(SlotIndex) as the placement rather than its raw ContainerSlot. Regression test
pins the shifted, gapless order. Full suite green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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