Somehow I've gotten the Table of Contents pane decoupled from (floating over) the book viewer window. How do I re-attache it as it is normally positioned on the left-hand side?
I tried simply dragging the contents pane to the top of the book viewer window and it does snap into position...full screen. Clicking resize in the upper right corner simply reproduces the floating ToC window. Something I've noticed, though: a button represented by three vertical dots that is normally present in the normally position-locked ToC window, on the right-hand edge of the window, is missing. If I recall correctly, clicking it made the ToC window slide into the left-hand window border, out of sight/hidden. All that functionality is gone.
Maybe a couple of images will make clear what I'm trying to describe:
https://i.imgur.com/V8lzUTy.png
https://i.imgur.com/3bM9lwl.png
Any insight would be greatly appreciated.