sortable: Small placeholders compute overlap wrong

When using sortables with much smaller drop-targets the computation of the overlay at which to switch the drop placeholder below the drop target is wrong.

It is not computed with the new location of the drop target, but instead with the old location of the drop target.

To clarify

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| first item in draggable list (item I want to drag)

| quite big

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|

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| second item (item I want to drag the first below) (quite small)

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The dropable placeholder is styled with css to be 10px tall, so it is much smaller than both the items, which themselves have different sizes. (other options are at their defaults, so overlap should be 50% to make the drop target jump below the original)

Now when I start dragging the first item, the second item moves up a great deal (due to the placeholder being very small) but to make the placeholder move below the dropped target you have to move the first item to a position where it would overlap the second item more than 50% at its location before the drag was started.

After this is done once, the overlap calculation works correctly - but is still repeatable when doing a fresh drag.