Up until a few days ago my Zotero standalone showed the folder tree on the left, the items (references) in a middle pane, and the item preview on the right. Now, all of the sudden, the preview pane has moved to a horizontal position below the references rather than a vertical one on their right side - a much inferior layout that hides most of the information about the item being previewed.

I know the panes can be collapsed, but I can't seem to drag it back to the desired position or find anything in the settings related to this. Help?