Thanks Lord, good try.

I finally solved it this morning.

In case this might help anyone else--The tiny sideways triangle buttons on the upper pane of the library window, the "shared files" pane, also at the top half of the window, on the right, and the two little tiny triangles that control the lower "playlist" pane interact in funny ways. NOT intuitively obvious. I clicked on some combination of these, I guess. I couldn't get the previous display back, though I messed around with them for some time.

This morning I messed around with these buttons some more, while dragging the bouncaries of the panes. The old configuration finally came back.

I hope this might be helpful to someone.