Hi there,
I have painfully experienced the same issue (among many), since November 2015 Update.
I only manage very large documents (reports, books) so book panels are my daily friends. Since November, when I stack a book panel in my workspace, then open an InDesign file from it, the panel instantly disappears behind the app frame. Therefore, I have to go to the Window menu to recover it, standing in kind of a ‘default’ and troublesome location (upper-left corner of the screen).
Since I used to quickly open/check/manipulate/close numerous documents of a book through this panel, the job is now extraordinary tedious with which appears to be a new version bug (stack the panel, lose it while opening a doc, go to the menu, move it back… 40 times a day).
I’m afraid we’ll have to wait until the Adobe staff discovers the problem and fix it. Maybe you could talk about it: that will surely speed the process!
Thanx.
François.