Dragged items can overlap

1. When using a multi-touch device, you can drag two items on top of one another
2. When a dragged item is being positioned into place, during its transition, you can drag another item on top of it.

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No file, test, or entry point is named in the issue. Start by reproducing both reported cases: dragging two items on a multi-touch device and dragging another item during a dragged item's transition. Done means neither interaction allows items to overlap, with regression coverage for both behaviors.

Written by the indexing model from the issue text.