Well at least we know the issue is confined to Auction lots and not every single window
I have completely eliminated the description field in Aw2000 here and I can re-drag any divider bar back whereever I want.
Have you tried clicking through the tabs on an Auction lot, Category, Item specifics and Shipping options, etc. And dragging any dividers that are draggable over a tad and then back. Then going back to the Description and Images field and see if the divider bar will drag and drop to reveal the description field?