I think I discovered an alternate solution.
But first I want to clarify the symptom the I've been experiencing...
In the Standard or Classic layout, when the preview pane is open and I drag the divider bar to close the pane the divider stops at a certain point. In Standard layout, it stops about 2-3" from the right edge of the window. In Classic layout, it stops about 1" from the bottom edge of the window. I can't make the divider go any farther, so I can't close the pane entirely. In addition, double-clicking the divider bar doesn't close the pane either (like it did in earlier versions of OS X.
The solution? Drag the divider bar all the way to the edge of the window. You won't see the divider bar move until your cursor is literally on the edge of the window, then... snap, the divider bar jumps to the edge of the window and the preview pane closes.
You don't need to do anything special in Preferences or switch layouts or anything. Simply drag all the way to the edge of the window, the divider bar will snap to the edge and the preview pane will close.
For reference, to open the preview pane, just drag the divider (that's on the edge of the window) away from the edge just a little and it will jump to the minimum distances mentioned above.
I hope this helps other Mac Mail users.