Edit Book: Blocked when saving and exiting...
Grab a big book (it's going to be easy to test).
Make any change so you can save. Save the book.
Quickly go and close the editor while the book is being saved. A warning appears.
Wait some time (just because you are possibly reading the warning). Because of this waiting, the background saving process will have already finished.
Select continuing the save process and quit.
Voilà. Editor is frozen with a "saving, please wait..." message in the middle.