Fix positioning on first draw of arrow theme

@adamschwartz 

This is why the drop elements weren't positioning correctly on the first draw.  You can't base the size of the element on it's attachment classes, as the attachment classes can only be added after we know the size of the element.

This change might have unintended consequences though, in terms of elements flipping `$arrowSize`px before they should, so I leave it for you to review.