Helper in a bottom-positioned Sortable is positioned wrong on drag start

When a Sortable is placed in an absolutely positioned container where the position is set using 'bottom' with all other default settings, the helper is not positioned properly on drag start. It is positioned too high vertically by the amount of the height of the helper. The reason for this bug is that the placeholder has not yet been created at the time that the parent offset is calculated, and the bottom alignment of the list causes the offset to be calculated as a smaller amount.

The fix for this bug is to create the placeholder before the offsets are calculated. Attached is an html file that shows the big reproducible, and a patch to fix the issue.